Fix test skipping for windows platform and avoid skipping MacOS

[NicolasHug]

The current condition for skipping tests on windows:

@unittest.skipIf('win' in sys.platform, 'temporarily disabled on Windows')

is also a match for macos ('darwin'): https://docs.python.org/3/library/sys.html#sys.platform

This PR fixes those so that the tests are only skipped on windows platforms.

[codecov]

Codecov Report

Merging #3548 (5b8cb3a) into master (9a08e47) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3548      +/-   ##
==========================================
- Coverage   78.86%   78.86%   -0.01%     
==========================================
  Files         105      105              
  Lines        9752     9750       -2     
  Branches     1567     1567              
==========================================
- Hits         7691     7689       -2     
  Misses       1574     1574              
  Partials      487      487              

Impacted Files	Coverage Δ
torchvision/datasets/mnist.py	55.21% <0.00%> (-0.39%)	⬇️

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[datumbox]

LGTM!