Update websockets library to work with Py3.9+

[johnsca]

Using websockets < 8.0 on Python >= 3.9 fails with a TypeError in the websockets library. However, version 8 of websockets drops support for older versions of Python that we still need to support, so this uses conditional dependencies to support both.

[SimonRichardson]

Does this also help with #461 (not checked the differences supporting 3 9+)?

[johnsca]

@SimonRichardson Alas, no. The loop arguments will still need to be explicitly removed before 3.10

[SimonRichardson]

!!build!!

Try again...

[SimonRichardson]

!!build!!

[SimonRichardson]

!!build!!

Sorry about the noise, just setting our jenkins CI python 3.9 ready.

[SimonRichardson]

@johnsca can you rebase your branch so we can land this.

[SimonRichardson]

!!build!!

[SimonRichardson]

@johnsca The reason the github-check-merge-juju-python-libjuju test is failing is that it can't find pylxd and we need this one to pass so I can get the merge bot to land it.

Not entirely sure this isn't being picked up for 3.9, but is for other versions.

09:22:21 ==================================== ERRORS ====================================
09:22:21 _______________ ERROR collecting tests/integration/test_model.py _______________
09:22:21 ImportError while importing test module '/home/ubuntu/go/src/github.com/juju/python-libjuju/tests/integration/test_model.py'.
09:22:21 Hint: make sure your test modules/packages have valid Python names.
09:22:21 Traceback:
09:22:21 /usr/lib/python3.9/importlib/__init__.py:127: in import_module
09:22:21     return _bootstrap._gcd_import(name[level:], package, level)
09:22:21 tests/integration/test_model.py:14: in <module>
09:22:21     import pylxd
09:22:21 E   ModuleNotFoundError: No module named 'pylxd'

[SimonRichardson]

$$merge$$