feat: remove Python3.7 support

[texastony]

Issue #, if available:

Description of changes:

• Stop testing or advertising support for py3.7
• Tell PyPi we support 3.12

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Check any applicable:

• [ ] Were any files moved? Moving files changes their URL, which breaks all hyperlinks to the files.

[texastony]

Also resolves https://github.com/aws/aws-dynamodb-encryption-python/issues/524