Description of changes:
bump required version of cryptography from 3.6.0. to 3.6.4, for consistency with other libraries.
update required version of cryptography mentioned in README to match
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.
Issue #, if available: #625
Description of changes: bump required version of cryptography from 3.6.0. to 3.6.4, for consistency with other libraries. update required version of cryptography mentioned in README to match
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.