Specify minimal version for cryptography package

[AmbientLighter]

Otherwise if pysaml is installed with an older release of cryptography package it would fail with
AttributeError: '_RSAPrivateKey' object has no attribute 'sign'

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-io]

Codecov Report

Merging #525 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #525      +/-   ##
==========================================
+ Coverage   65.13%   65.19%   +0.05%     
==========================================
  Files         100      100              
  Lines       25693    25693              
==========================================
+ Hits        16735    16750      +15     
+ Misses       8958     8943      -15

Impacted Files	Coverage Δ
src/saml2/validate.py	76.86% <0%> (+5.88%)	⬆️

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