Deprecating Python 3.4 and 3.5 support (tracking issue)

[LinuxMercedes]

We have a few packages that we may want to upgrade in the future, but doing so will require dropping support for unsupported Python versions. This keeps track of them.

• Pylint: version 2.7.0, which fixes the issue worked around in 27ef2bf, does not support 3.4 or 3.5. Since this is a dev dependency, not a big deal, just requires more environment config for the CI tests.
• GitPython: future versions will drop python 3.4 support; we'll want to upgrade when a fix for GitPython can stage changes for commit that git won't commit gitpython-developers/GitPython#1185 lands.
• Dropping support for 3.4 will also let us use more typing checks.

[LinuxMercedes]

We also need to upgrade off of nose, as it's been unmaintained since 2016.