INTPYTHON-841 Remove py39 fallback from typing.TypeAlias

[caseyclements]

INTPYTHON-841

Summary

One line change to remove fallback to get TypeAlias from typing_extensions for python 3.9.

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is the intention of the code captured in relevant tests?
• If there are new TODOs, has a related JIRA ticket been created?
• Has a MongoDB Employee run the patch build of this PR?

Checklist for Reviewer {@primary_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?)
• Have you checked for spelling & grammar errors?
• Is all relevant documentation (README or docstring) updated?

Focus Areas for Reviewer (optional)

Evergreen Patch Link