Fix various files so the entire trio/ directory can be type-checked

[jakkdl]

This is a squash of #2721 to avoid having to navigate a huge and messy PR. It consists of a ton of small fixes or type: ignores to any file that raised errors when running mypy trio/, and it also transitions pyproject.toml from being a list of files to check strictly, to an ignore-list with files to disable strict checks for. This doubles as a TODO-list, and it's easy to explode wildcards, or remove a disabled check, or similar.

The test changes has caught several small typing errors introduced in various PRs that haven't been checked in CI, or when files have been forgotten to get added.

All the changes should be non-controversial, if anything is up for debate I'll just mark it with a comment and/or revert it, and add it to #2734 or directly split out to a separate PR

[codecov]

Codecov Report

Merging #2763 (cb6214e) into master (4c38ba9) will increase coverage by 0.01%.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2763      +/-   ##
==========================================
+ Coverage   98.92%   98.93%   +0.01%     
==========================================
  Files         113      113              
  Lines       16728    16752      +24     
  Branches     3026     3027       +1     
==========================================
+ Hits        16548    16574      +26     
+ Misses        124      123       -1     
+ Partials       56       55       -1     

Files Changed	Coverage Δ
trio/_tools/gen_exports.py	100.00% <ø> (ø)
trio/_core/_io_common.py	100.00% <100.00%> (ø)
trio/_core/_ki.py	100.00% <100.00%> (ø)
trio/_core/_parking_lot.py	100.00% <100.00%> (ø)
trio/_core/_tests/test_io.py	100.00% <100.00%> (ø)
trio/_core/_tests/test_ki.py	97.82% <100.00%> (+0.01%)	⬆️
trio/_core/_tests/test_multierror.py	100.00% <100.00%> (ø)
trio/_core/_tests/test_run.py	100.00% <100.00%> (ø)
trio/_core/_wakeup_socketpair.py	100.00% <100.00%> (ø)
trio/_path.py	100.00% <100.00%> (ø)
... and 12 more

[jakkdl]

without (possibly a subset of) the changes to _fake_net:

trio/testing/_fake_net.py:107: error: Signature of "socket" incompatible with supertype "SocketFactory"  [override]
trio/testing/_fake_net.py:107: note:      Superclass:
trio/testing/_fake_net.py:107: note:          def socket(self, family: Union[AddressFamily, int, None] = ..., type: Union[SocketKind, int, None] = ..., proto: Optional[int] = ...) -> _SocketType
trio/testing/_fake_net.py:107: note:      Subclass:
trio/testing/_fake_net.py:107: note:          def socket(self, family: int, type: int, proto: int) -> FakeSocket
trio/testing/_fake_net.py:116: error: Argument 1 of "getaddrinfo" is incompatible with supertype "HostnameResolver"; supertype defines the argument type as "Union[bytes, str, None]"  [override]
trio/testing/_fake_net.py:116: note: This violates the Liskov substitution principle
trio/testing/_fake_net.py:116: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
trio/testing/_fake_net.py:116: error: Argument 2 of "getaddrinfo" is incompatible with supertype "HostnameResolver"; supertype defines the argument type as "Union[bytes, str, int, None]"  [override]
trio/testing/_fake_net.py:131: note: By default the bodies of untyped functions are not checked, consider using --check-untyped-defs  [annotation-unchecked]
trio/testing/_fake_net.py:179: error: Need type annotation for "_packet_sender"  [var-annotated]
trio/testing/_fake_net.py:179: error: Need type annotation for "_packet_receiver"  [var-annotated]
Found 5 errors in 1 file (checked 129 source files)

[jakkdl]

merging this one so any additional minor problem in other typing PRs can be caught, gonna lead to a bunch of merge conflicts in pyproject.toml either way