Update threading.Event.isSet() calls

[The-Compiler]

The new is_set() snake-case spelling has been available since
Python 2.6:
https://docs.python.org/2/library/threading.html#threading.Event.is_set

The old isSet() camel-case spelling raises a deprecation warning with
Python 3.10:
https://docs.python.org/3.10/whatsnew/3.10.html#deprecated
(almost at the end)

[RonnyPfannschmidt]

@The-Compiler pipeline failure is related to lacking isolation and a setuptools bug

quickfix should be throwing a pip install in for setuptools_scm before the tox run and maybe into the deps section

[The-Compiler]

I won't have time to fix that one I'm afraid, I'm already busy with chasing some ~5 issues related to pytest and Python 3.10 (this being one of them).

[RonnyPfannschmidt]

@The-Compiler i'll take a look a fixing the pr in my next time slot then

[The-Compiler]

The PR isn't broken though, the pipeline is - why not just merge this and drop support for Python 2.7 and 3.5 (which are the only failing environments)?

[RonnyPfannschmidt]

@The-Compiler i'll take a few to coordinate that, i like the suggestion