Use cryptography.x509 for certificate loading

[ioparaskev]

• Uses cryptography.x509 load_pem_x509_certificate or
  load_der_x509_certificate depending on the cert type. This ensures 1)
  the certificate is a valid certificate 2) trailing newlines and
  whitespaces will be ignored
• Adds tests for the above scenarios
• Ignores cer/crt as certificate type since these are file extensions
  and do not guarrantee the certificate encoding. Uses "pem" as default
  type (mostly for backward compatibility). Only other valid option is
  "der" (everything else falls back to "pem")
• Uses sigver cert loading function for the metadata cert loading
  (removes read_cert function)
• Fixes x509 certificate with trailing whitespace doesn't load #659

All Submissions:

• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• Have you added an explanation of what problem you are trying to solve with this PR?
• Have you added information on what your changes do and why you chose this as your solution?
• Have you written new tests for your changes?
• Does your submission pass tests?
• This project follows PEP8 style guide. Have you run your code against the 'flake8' linter?

[codecov]

Codecov Report

Merging #660 into master will increase coverage by <.01%.
The diff coverage is 90.47%.

@@            Coverage Diff             @@
##           master     #660      +/-   ##
==========================================
+ Coverage   65.06%   65.06%   +<.01%     
==========================================
  Files         102      102              
  Lines       25667    25659       -8     
==========================================
- Hits        16700    16696       -4     
+ Misses       8967     8963       -4

Impacted Files	Coverage Δ
src/saml2/sigver.py	70.45% <100%> (+0.22%)	⬆️
src/saml2/metadata.py	45.06% <71.42%> (-0.11%)	⬇️

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[peppelinux]

Is there any possibility to handle also #278 in this PR?

[c00kiemon5ter]

I don't think we should handle #278 here. I also don't certificate-passwords are widely used. Let's keep this issue about loading certificates only and we can revisit certificate-passwords again in the future in a different issue/PR.

[peppelinux]

I marked that as closing and then closed It, now I've reopened it again. I have a presentiment that It would be a "won't fix" but my excuse in Advance, I didn't mean to be cynical

[peppelinux]

I pushed this feature here
daf2142#diff-3482e02155ca5e77a0034bd27c342d3951400f11ceea80ed7d2169e8f458f857R105

Probably it would belong to this PR.
Don't know which PR would be merged first, so, let me know your though, eventually I can push it also in this PR

[c00kiemon5ter]

closed by a924435