Update to use sha256 instead of md5

[mamccorm]

Updating several references to md5, to use a more secure hash such as sha256.

Checklist:

• Add tests that demonstrate the correct behavior of the change. Tests should fail without the change.
• Add or update relevant docs, in the docs folder and in code.
• Add an entry in CHANGES.rst summarizing the change and linking to the issue.
• Add .. versionchanged:: entries in any relevant code docs.
• Run pre-commit hooks and fix any issues.
• Run pytest and tox, no tests failed.

[john0isaac]

This doesn't update the docstrings and tests.