setup.py claims support for Python 2 but code actually requires Python 3

[FelixSchwarz]

Your setup.py claims Python 2 support but TrackedList.sort() uses a keyword-only parameter which is only valid for Python 3.

Probably Python 2 support is a non-goal by now so I suggest:

• fix setup.py
• add `python_requires = ">= 3.6"

If you like I can create a pull request to fix this issue.

[edelooff]

That's a good point, and yes, Python 2 compatibility is no longer a goal. Happy to accept the PR.