Drop 3.8 support

[CoolCat467]

Closes #3104, this pull request aims to handle dropping python 3.8 support and the various changes we can now make with that.

One thing I'm not sure how to handle, see
src/trio/_tests/test_path.py:81

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.58%. Comparing base (c47758c) to head (507dee3).
Report is 18 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3106   +/-   ##
=======================================
  Coverage   99.58%   99.58%           
=======================================
  Files         121      121           
  Lines       18155    18167   +12     
  Branches     3270     3268    -2     
=======================================
+ Hits        18080    18092   +12     
  Misses         52       52           
  Partials       23       23           

Files with missing lines	Coverage Δ
src/trio/_channel.py	100.00% <ø> (ø)
src/trio/_core/_asyncgens.py	100.00% <ø> (ø)
src/trio/_core/_entry_queue.py	100.00% <100.00%> (ø)
src/trio/_core/_instrumentation.py	100.00% <100.00%> (ø)
src/trio/_core/_io_kqueue.py	87.20% <100.00%> (ø)
src/trio/_core/_io_windows.py	98.80% <ø> (ø)
src/trio/_core/_tests/test_guest_mode.py	100.00% <100.00%> (ø)
src/trio/_core/_tests/test_instrumentation.py	100.00% <100.00%> (ø)
src/trio/_core/_tests/test_io.py	100.00% <100.00%> (ø)
src/trio/_core/_tests/test_ki.py	97.83% <100.00%> (ø)
... and 27 more

[TeamSpen210]

Noticed something that's technically 3.9 incompatible, but probably not in practice. For test_path, you'll just want to add (pathlib.Path, trio.Path) so that combo gets tested.

[CoolCat467]

For test_path, you'll just want to add (pathlib.Path, trio.Path) so that combo gets tested.

Currently fails because of assert that return type is trio.Path

[A5rocks]

Once again ran into the bad cache for CI jobs thing. I really wonder what could be causing it; seems to be some pip thing?

[A5rocks]

Basically just nitpicks.

[jakkdl]

I suspect most from __future__ import annotations could be removed, but /shrug