Jenkins tests now require python 3.6+

Hi lovely sysadmins. Stem 1.8 is the last version that will support Python 2.6 through 3.5. Going forward Stem's upcoming 2.x series will require Python 3.6...

https://gitweb.torproject.org/stem.git/commit/?id=a396f57

As such our Jenkins tests now fail with...

Traceback (most recent call last):
  File "./run_tests.py", line 22, in <module>
    import stem.prereq
  File "/srv/jenkins-workspace/workspace/stem-tor-ci/stem/__init__.py", line 1032, in <module>
    import stem.util.connection  # importing afterward to avoid circular dependency
  File "/srv/jenkins-workspace/workspace/stem-tor-ci/stem/util/connection.py", line 65, in <module>
    import urllib.request
ImportError: No module named request

Could we please upgrade their Python version to 3.6 (or better, 3.7)?

Thanks!

PS. In the meantime Jenkins can test git's 1.8.0 tag rather than the master branch if you'd like for the tests to resume passing in the short term.