Bump the minimum attrs ver for @frozen

[FasterSpeeding]

I've run MyPy and pytest against this locally with the new minimum attrs version installed to make sure this bump fixes the problem.

Would've waited until #2449 was merged to open this but that seems a bit stalled.

Fixes: #2439

[codecov]

Codecov Report

Merging #2499 (6fb73ea) into master (f39d3a4) will increase coverage by 0.86%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2499      +/-   ##
==========================================
+ Coverage   97.98%   98.84%   +0.86%     
==========================================
  Files         118      118              
  Lines       16324    16324              
  Branches     3156     3156              
==========================================
+ Hits        15995    16136     +141     
+ Misses        266      133     -133     
+ Partials       63       55       -8     

Impacted Files	Coverage Δ
trio/_core/_run.py	99.45% <0.00%> (+0.43%)	⬆️
trio/tests/test_socket.py	99.83% <0.00%> (+0.64%)	⬆️
trio/_core/tests/test_io.py	100.00% <0.00%> (+1.05%)	⬆️
trio/tests/test_highlevel_open_tcp_stream.py	100.00% <0.00%> (+1.47%)	⬆️
trio/_subprocess_platform/__init__.py	100.00% <0.00%> (+5.71%)	⬆️
trio/_core/__init__.py	100.00% <0.00%> (+12.50%)	⬆️
trio/lowlevel.py	100.00% <0.00%> (+16.66%)	⬆️
trio/_core/_io_kqueue.py	84.67% <0.00%> (+83.87%)	⬆️
trio/_subprocess_platform/kqueue.py	100.00% <0.00%> (+100.00%)	⬆️

[A5rocks]

Looks good, just leaving a note here that while attr.frozen was added in 20.1.0, that was only provisionally (it was stablized seemingly in 21.1.0). That shouldn't affect anything, of course. I don't think there were any API changes, just bugfixes.

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[harahu]

@FasterSpeeding thank you for the contribution 🥳