Merge python 3 fixes with offer-list fixes.

Part of 28863.

Change "source=whitelist" to "source=offer-list".

Part of 32699 and 24839.

The input list is a list of potential fallbacks, that relay operators
have offered to have hard-coded in Tor. To avoid relay operator
confusion, we should rename the list based on how we use it.

Preparation for 24839.

Fixes bug 32699.

Reduce the number of connections in generate.
Simplify and remove compatibility code.
Increase retries, and add a timeout.

Part of 28863.

Part of 28863.

Part of 28863.

Part of 28863.

Part of 28863.

Part of 28863.

Part of 28863.

Part of 28863.

Part of 28863.

But do replacements for decoding errors.

Part of 28863.

Part of 28863.

Part of 28863.

Part of 28863.

Rather than just printing "None".

Part of 28863.

Some relays are unreliable, but we still want the tests to check
the different types of relays.

Part of 28863.

And move a comment.

Part of 28863.

Part of 28863.

But maintain compatibility with older stem versions.

Part of 28863.

Comment-only change.

Part of 28863.

Part of 28863.

This change makes it easier to run the travis tests locally.

Part of 28863.

Part of 32632 and 28863.

Part of 28863.

Part of 28863.

Part of 28863.

Part of 28863.

Use Xenial for pypy.

Part of 32632 and 28863.

Use Ubuntu Bionic and macOS 10.14.

Install Homebrew python@2 in macOS
macOS needs pip(2) to install fallback scripts dependencies.

Part of 32632 and 28863.

* Restrict the valid True and False bool values
* use default for unset env var, and (not default) for empty set env var

Closes 31305.

Part of 31305.