Run tests on macOS in CI, and some fixes to get it in reasonable state

[itamarst]

I filed #935 as eventual follow-up to get a few tests actually passing, but this PR is an improvement on the status quo, at least.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 56%. Comparing base (1f3db00) to head (415d2fe).

Additional details and impacted files

@@          Coverage Diff          @@
##           master   #934   +/-   ##
=====================================
  Coverage      56%    56%           
=====================================
  Files          89     89           
  Lines        9748   9751    +3     
  Branches     1812   1812           
=====================================
+ Hits         5471   5476    +5     
  Misses       3905   3905           
+ Partials      372    370    -2     

Flag	Coverage Δ
ipv6	23% <33%> (+<1%)	⬆️
py310asyncio	53% <100%> (+<1%)	⬆️
py310epolls	53% <33%> (-1%)	⬇️
py310poll	53% <33%> (-1%)	⬇️
py310selects	53% <33%> (-1%)	⬇️
py311asyncio	53% <100%> (+<1%)	⬆️
py311epolls	53% <33%> (-1%)	⬇️
py312asyncio	50% <100%> (+<1%)	⬆️
py312epolls	51% <33%> (+<1%)	⬆️
py37asyncio	50% <100%> (-1%)	⬇️
py37epolls	51% <33%> (-1%)	⬇️
py38asyncio	51% <100%> (-1%)	⬇️
py38epolls	53% <33%> (-1%)	⬇️
py38openssl	52% <33%> (-1%)	⬇️
py38poll	53% <33%> (-1%)	⬇️
py38selects	53% <33%> (+<1%)	⬆️
py39asyncio	51% <100%> (-1%)	⬇️
py39dnspython1	51% <33%> (-1%)	⬇️
py39epolls	53% <33%> (-1%)	⬇️
py39poll	53% <33%> (-1%)	⬇️
py39selects	53% <33%> (-1%)	⬇️

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[itamarst]

This should probably be merged before #930