Remove obsolete pycrypto dependency

[exarkun]

Fixes ticket:2873

[codecov-io]

Codecov Report

Merging #514 into master will decrease coverage by 0.34%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #514      +/-   ##
==========================================
- Coverage   90.35%   90.01%   -0.35%     
==========================================
  Files         144      143       -1     
  Lines       27466    26267    -1199     
  Branches     3939     3699     -240     
==========================================
- Hits        24818    23643    -1175     
+ Misses       1924     1905      -19     
+ Partials      724      719       -5

Impacted Files	Coverage Δ
src/allmydata/_auto_deps.py	100% <ø> (ø)	⬆️
src/allmydata/util/fileutil.py	50.56% <0%> (-5.65%)	⬇️
src/allmydata/scripts/tahoe_ls.py	82.46% <0%> (-5.2%)	⬇️
src/allmydata/util/iputil.py	64.7% <0%> (-3.37%)	⬇️
src/allmydata/immutable/downloader/share.py	92.82% <0%> (-2.47%)	⬇️
src/allmydata/dirnode.py	96.52% <0%> (-1.74%)	⬇️
src/allmydata/__init__.py	69.89% <0%> (-1.39%)	⬇️
src/allmydata/util/encodingutil.py	89% <0%> (-1.05%)	⬇️
src/allmydata/immutable/checker.py	87.43% <0%> (-1.03%)	⬇️
src/allmydata/mutable/publish.py	95.63% <0%> (-1.02%)	⬇️
... and 6 more

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[exarkun]

Slackware failure is known and fixed by #515
Fedora failure is the same old test_system port collision failure.
No idea what the deal with the codecov failures are. This clearly does not remove any meaningful test coverage.

[exarkun]

@MostAwesomeDude gave this a thumbs up on IRC.