fix: hard set cryptography dependency

[chamcca]

Issue #, if available: Fixes #85

Description of changes:

• update requirements.txt to hard set cryptography dependency to major/minor version 37.0 as latest seems to have changed the interfaces

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[mrinaudo-aws]

Hi @chamcca - thank you for your time. See my comments in the relevant issue here, whereas this is an issue not directly related to this sample and should not be fixed here. Closing the pull request.