Switch to using trustme for testing TLS certs

[njsmith]

Don't merge this yet - it needs
python-trio/trustme#4
to be merged and released first.

I guess this won't be a problem b/c all the tests are going to fail
mightily until that happens. But I figured I'd get put the branch up
in any case so it doesn't get misplaced.

[njsmith]

New trustme is released; let's see how this goes.

[njsmith]

Ok, did another release of trustme, now actually testing on Windows. Maybe this will work better.

[codecov]

Codecov Report

Merging #244 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #244      +/-   ##
==========================================
+ Coverage   99.11%   99.14%   +0.02%     
==========================================
  Files          63       63              
  Lines        8739     8850     +111     
  Branches      627      633       +6     
==========================================
+ Hits         8662     8774     +112     
  Misses         61       61              
+ Partials       16       15       -1

Impacted Files	Coverage Δ
trio/tests/test_ssl.py	100% <100%> (ø)	⬆️
trio/_core/_local.py	100% <0%> (ø)	⬆️
trio/tests/test_socket.py	100% <0%> (ø)	⬆️
trio/abc.py	100% <0%> (ø)	⬆️
trio/_core/tests/test_local.py	100% <0%> (ø)	⬆️
trio/socket.py	100% <0%> (+0.43%)	⬆️

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