PYTHON-5467 Add codecov integration

[blink1073]

PYTHON-5467

Changes in this PR

Add codecov integration for pymongo. The GitHub Action portion ties the coverage to a PR, and the evergreen portion adds coverage data for other variants. Adds coverage to the No C extensions variant to demonstrate the coverage contributions from the Evergreen builds, getting the bson/__init__.py coverage up to 77%. Here's the report for now. Once this is merged, the PR comment will look more like this on in the PHP driver, that updates as new reports are uploaded.

Test Plan

Test codecov interaction with the PR.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is there test coverage?
• Is any followup work tracked in a JIRA ticket? If so, add link(s).

Checklist for Reviewer

• Does the title of the PR reference a JIRA Ticket?
• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Is all relevant documentation (README or docstring) updated?

[codecov-commenter]

Welcome to Codecov 🎉

Once you merge this PR into your default branch, you're all set! Codecov will compare coverage reports and display results in all future pull requests.

Thanks for integrating Codecov - We've got you covered ☂️