chore: use cryptography package instead of openssl.crypto in tests

[ThosRTanner]

Fixes #782

Why is this needed?

Stops us moving to later versions of openssl module

Proposed Changes

• Rip out all of openssl.crypto logic in testing/common.py and replace with cryptography calls

Does this PR introduce any breaking change?

No - this only affects testing

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.96%. Comparing base (79cae1f) to head (7f2db60).
⚠️ Report is 10 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #823   +/-   ##
=======================================
  Coverage   94.96%   94.96%           
=======================================
  Files          27       27           
  Lines        3811     3811           
=======================================
  Hits         3619     3619           
  Misses        192      192           

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