Fix a bunch of ResourceWarnings due to leaked fds

[njsmith]

Prompted by this random test failure:
#200 (comment)

[codecov]

Codecov Report

Merging #1565 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1565   +/-   ##
=======================================
  Coverage   99.68%   99.68%           
=======================================
  Files         110      110           
  Lines       13466    13478   +12     
  Branches     1024     1026    +2     
=======================================
+ Hits        13423    13435   +12     
  Misses         28       28           
  Partials       15       15           

Impacted Files	Coverage Δ
trio/_highlevel_open_tcp_stream.py	96.96% <ø> (ø)
trio/_core/tests/test_multierror.py	100.00% <100.00%> (ø)
trio/tests/test_highlevel_open_tcp_listeners.py	100.00% <100.00%> (ø)
trio/tests/test_highlevel_socket.py	100.00% <100.00%> (ø)
trio/tests/test_highlevel_ssl_helpers.py	100.00% <100.00%> (ø)
trio/tests/test_socket.py	100.00% <100.00%> (ø)
trio/tests/test_ssl.py	99.86% <100.00%> (+<0.01%)	⬆️

[pquentin]

Thanks!