TST: Properly test with pymongo, adjust profiler usage for py3.12

[tangkong]

Description

• Properly mark test_multi_backend as needing pymongo. (its fixtures had this decorator already, but the test still tried to run 🤷 )

Motivation and Context

Seen in #343 and others

How Has This Been Tested?

CI

Where Has This Been Documented?

This PR

Pre-merge checklist

• Code works interactively
• Code contains descriptive docstrings, including context and API
• New/changed functions and methods are covered in the test suite where possible
• Test suite passes locally
• Test suite passes on GitHub Actions
• Ran docs/pre-release-notes.sh and created a pre-release documentation page
• Pre-release docs include context, functional descriptions, and contributors as appropriate

[tangkong]

Python 3.12 slightly changed how the built-in cprofiler behaves. Now the following raises an error:

import cProfile

p1 = cProfile.Profile()
p1.enable()

p2 = cProfile.Profile()
# previously, p1 is silently disabled here, now python raises
p1.disable()
p2.enable()

(a very helpful comment)

The py3.12 tests only fail on the cprofile test cases, which is consistent with this issue