Maintenance: apply pyupgrade suggestions for py3.9

[jayaddison]

Feature or Bugfix

• Refactoring

Purpose

• Some suggestions provided by pyupgrade to modernise the codebase to the Python 3.9 baseline.

Detail

• For application code: find sphinx -name '*.py' -exec pyupgrade --py39 {} \;
• For the test suite: began by running a similar command, but that resulted in many false positives. The test suite makes use of various pre-py3.9 patterns that we should likely retain support for for a while.

Relates

• N/A

[jayaddison]

Some caution here: it looks like I used the wrong commandline invocation. That should have been:

$ find sphinx -name '*.py' -exec pyupgrade --py39-plus {} \;

The results seem no different, but I'm double-checking it anyway.

[jayaddison]

Some caution here: it looks like I used the wrong commandline invocation.

In this case, this was a non-issue, thanks to the argparse setting allow_abrev having a default of True - unambiguous long-name flags/options are expanded.

As it turns out, all of the changes here seem acceptable for py3-only too. But I'll keep the pull request description and title as-is, based on the intent.

[picnixz]

Thank you !