Bump cryptography version ceiling from <50 to <51 in setup.cfg

[RyAuld]

Summary

The \cryptography\ package has released version 48.0.0. Per the project's deprecation policy, the upper bound in \setup.cfg\ should be set to N+3 (i.e. <51).

Current value in \setup.cfg:
\
cryptography>=2.5,<50
\\

Should be updated to:
\
cryptography>=2.5,<51
\\

The test \ est_ceiling_should_be_latest_cryptography_version_plus_three\ in \ ests/test_cryptography.py\ validates this.