fix: CI: Test all supported python versions

As in chutney and stem:
- Test all supported python versions
- Test all supported tor versions

Differences between chutney, stem and sbws:
- in sbws we run directly, not an script that calls tox
- we're not using chutney for integration tests (yet) and therefore we're not testing it with different networks
- we don't have shellcheck tests
- we don't support osx nor windows

Closes: #30067

.travis.yml

-sudo: true
-dist: xenial
 language: python
-python:
-  - "3.5"
-  - "3.6"
-install: pip install tox-travis
+# The default python version on Travis bionic is 3.6
+# But we add this line to show the python version in the Travis UI
+python: "3.6"
+
+env:
+  matrix:
+    ## This matrix entry is required, but it doesn't actually create any jobs
+    ## by itself. All jobs are created by matrix: include: entries
+    ##
+    ## The TOR env var should be kept in sync with the Linux tor version in
+    ## the addons section below
+    - TOR="master-nightly"
+
+matrix:
+  # include creates Linux, python 3.6, tor master builds by default
+  # we use tor master to catch tor issues before stable releases
+  # the key(s) in each item override these defaults
+  include:
+    ## Test all supported and available tor versions on Linux
+    ## If the deb.torproject.org repositories are removed, we will fall back to
+    ## Ubuntu security's tor version (currently 0.2.9.14). We might want to
+    ## automatically fail the job if we can't get a newer tor, see #29741.
+    ##
+    ## The current tor versions in Ubuntu are on this page:
+    ## https://packages.ubuntu.com/search?keywords=tor&searchon=names&exact=1
+    ##
+
+    - addons:
+        apt:
+          sources:
+            - sourceline: 'deb https://deb.torproject.org/torproject.org tor-nightly-0.3.5.x-bionic main'
+              key_url: 'https://deb.torproject.org/torproject.org/A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89.asc'
+          packages:
+            - tor
+      env: TOR="0.3.5-nightly"
+    - addons:
+        apt:
+          sources:
+            - sourceline: 'deb https://deb.torproject.org/torproject.org tor-nightly-0.4.0.x-bionic main'
+              key_url: 'https://deb.torproject.org/torproject.org/A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89.asc'
+          packages:
+            - tor
+      env: TOR="0.4.0-nightly"
+    ## The current stable release is 0.4.1
+    - addons:
+        apt:
+          sources:
+            - sourceline: 'deb https://deb.torproject.org/torproject.org bionic main'
+              key_url: 'https://deb.torproject.org/torproject.org/A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89.asc'
+          packages:
+            - tor
+      env: TOR="stable-release"
+    ## We also have 0.4.1 and 0.4.2 nightlies
+    - addons:
+        apt:
+          sources:
+            - sourceline: 'deb https://deb.torproject.org/torproject.org tor-nightly-0.4.1.x-bionic main'
+              key_url: 'https://deb.torproject.org/torproject.org/A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89.asc'
+          packages:
+            - tor
+      env: TOR="0.4.1-nightly"
+    - addons:
+        apt:
+          sources:
+            - sourceline: 'deb https://deb.torproject.org/torproject.org tor-nightly-0.4.2.x-bionic main'
+              key_url: 'https://deb.torproject.org/torproject.org/A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89.asc'
+          packages:
+            - tor
+      env: TOR="0.4.2-nightly"
+
+    ## Test all supported python releases
+
+    ## Pre-installed in Travis Bionic:
+    ## https://docs.travis-ci.com/user/reference/bionic/#python-support
+
+    ## End of Life: December 2021
+    ## https://www.python.org/dev/peps/pep-0494/#lifespan
+    - python: "3.6"
+
+    ## End of Life: June 2023
+    ## https://www.python.org/dev/peps/pep-0537/#lifespan
+    - python: "3.7"
+
+    ## Extra Installs
+
+    ## End of Life: October 2024
+    ## https://www.python.org/dev/peps/pep-0569/#lifespan
+    - python: "3.8"
+
+    ## Python 3.9
+    ## (Add 3.9-dev), so far is the same as nightly
+    ## Stable: 10 October 2020
+    ## (Switch from 3.9-dev to 3.9, and check for {3.10,4.0}-dev)
+    ## End of Life: October 2025
+    ## https://www.python.org/dev/peps/pep-0596/#lifespan
+
+    - python: "nightly"
+
+  allow_failures:
+    # stem fails:
+    # <ERROR> Error initting controller socket: module 'collections' has no
+    # attribute 'Iterable'
+    - python: nightly
+
+## (Linux only) Use the Ubuntu Bionic Linux Image
+dist: bionic
+
+## Download our dependencies
+addons:
+  ## (Linux only)
+  apt:
+    sources:
+      - sourceline: 'deb https://deb.torproject.org/torproject.org tor-nightly-master-bionic main'
+        key_url: 'https://deb.torproject.org/torproject.org/A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89.asc'
+    packages:
+      - tor
+      # To build the docs
+      - texlive-latex-extra
+      - dvipng
+
+install:
+  - pip install tox-travis
+  - dpkg-query --show
+  ## Use the default spelling for python, unless it is overridden
+  - export PYTHON=${PYTHON:-python}
+  - $PYTHON --version
+  - tor --version
+  - tox --version
+
 script:
   - tox
   # This is not in included in the tox envlist, in order to don't need Internet
@@ -12,12 +135,6 @@ script:
   # - tox -e doclinks
   - tox -e clean
 
-before_install:
-    - gpg --version
-    - sudo apt-key add tests/deb.torproject.org.asc
-    - echo "deb https://deb.torproject.org/torproject.org xenial main" | sudo tee -a /etc/apt/sources.list
-    - sudo apt-get update -qq
-    - sudo apt-get install tor -y
 
 notifications:
   irc:
@@ -26,10 +143,3 @@ notifications:
     template:
       - "%{repository_slug} %{branch} %{commit} - %{author}: %{commit_subject}"
       - "Build #%{build_number} %{result}. Details: %{build_url}"
-
-# To build the docs
-addons:
-  apt:
-    packages:
-    - texlive-latex-extra
-    - dvipng

tox.ini

 [tox]
 skip_missing_interpreters = True
-envlist = py35, py36, inst, setup, unit, integration, lint, stats, doc
+envlist = py{36, 37, 38, 39, nightly}, inst, setup, unit, integration, lint, stats, doc
 
 [travis]
 python =
-    3.5: py35, inst, setup, unit, integration, lint, doc
-    3.6: py36, inst, setup, unit, integration
+    3.6: py36, inst, setup, unit, integration, lint, doc
+    3.7: py37, inst, setup, unit, integration, lint, doc
+    3.8: py38, inst, setup, unit, integration, lint, doc
+    3.9-dev: py39, inst, setup, unit, integration, lint, doc
+    nightly: pynightly, inst, setup, unit, integration, lint, doc
 
 [testenv]
 # install_command can be removed when --process-dependency-links is not