add to_cryptography/from_cryptography on CRL and X509Req

[reaperhulk]

Fixes #267

[codecov]

Codecov Report

Merging #645 into master will increase coverage by 0.02%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #645      +/-   ##
==========================================
+ Coverage   96.86%   96.88%   +0.02%     
==========================================
  Files          18       18              
  Lines        5644     5686      +42     
  Branches      389      391       +2     
==========================================
+ Hits         5467     5509      +42     
  Misses        119      119              
  Partials       58       58

Impacted Files	Coverage Δ
tests/test_crypto.py	98.6% <100%> (+0.01%)	⬆️
src/OpenSSL/crypto.py	96.65% <100%> (+0.05%)	⬆️

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