various small fixes

[jakkdl]

Various small fixes I noticed while going through code/doc/issues.

• doc updates to 113 and startable-in-context-manager
• ASYNC102 await-in-finally-or-cancelled no longer treats open_nursery as a cancel scope.
  • While you can make the cancelscope inside the nursery shielded with a timeout, that was never actually supported (there was no logic for seeing nursery.cancelscope.shield = True, only for nursery.shield = True, which is not possible)
  • open_nursery is itself an async function so you can't safely open a nursery anyway.
• async112 error message now specifies if it's a nursery or taskgroup.
  • for dumb reasons I confused myself into thinking async111 & async112 didn't have asyncio support, and fully rewrote that from scratch :upside_down_face: But some of the stuff was better so I kept it after noticing my mistake and rebasing onto an updated main.

[jakkdl]

Don't think this needs a version/changelog bump, but it would probably be good to have some check for if the changelog is updated in a pull request as that's been missed in the past.

[jakkdl]

I think the coverage failure on py39 is a bug in pytest-cov or coverage (??). I can reproduce it locally when running through tox. It's triggering here on line 130: https://github.com/python-trio/flake8-async/blob/f383fe696129526ff339060460535c826c51d7fd/flake8_async/visitors/visitors.py#L129-L130

• inserting a print statement before the continue has the result printed, and the continue statement (now on L131) is now covered
• when inserting a pass statement on L130, I get cov fail on L131 (where the continue now is). pass might be special-cased though idk.
  • I get same behaviour if inserting "hello", or assert True on L130
• it does not manifest on py310 or later
• removing --cov-branch made no difference
• I tried downgrading both pytest-cov and coverage, but going as far back as pytest-cov<4 and coverage<7 made no difference
• unsurprisingly it's not pytest-cov, running pytest directly with coverage run -m pytest ... made no difference

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