fix: update pyca range

[texastony]

Issue #, if available: #307

Description of changes:

• Update cryptography dependency range.

Testing:
I tested every Major Pyca since 2.5.0 explicitly via:

1. sed -i.backup 's/cryptography>=2.5.0/cryptography==$PYCA/g' requirements.txt
2. tox -e py38-integ -r
3. git restore requirements.txt
   Where PYCA is one of [2.5.0, 3.0.*, 3.2.*, 3.3.*, 35.*, 36.*, 37.*, 3.4.*].
   Only 37 failed.

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