Fix compatibility with Python 3.13

[c00kiemon5ter]

Closes #976

Continuation of #978

Description

I recently wrote a blog post demonstrating how to use Django as a service provider, with pysaml2 serving as an example IdP. A reader has since pointed out that it doesn’t work with Python 3.13.

The feature or problem addressed by this PR

After some investigation, I found two issues:

• removal of the cgi module in Python 3.13 Python 3.13 Compatibility #976
• dbm.sqlite being the new default dbm backend introduced in Python 3.13

What your changes do and why you chose this solution

This PR removes remaining usages of the deprecated cgi module and uses dbm.gnu as the default dbm backend which is used by _shelve_compat.

Checklist

• Checked that no other issues or pull requests exist for the same issue/change
• Added tests covering the new functionality
• Updated documentation OR the change is too minor to be documented
• Updated CHANGELOG.md OR changes are insignificant