PYTHON-5639 Prep for 4.15.4 release

[blink1073]

Signed-off-by: mongodb-dbx-release-bot[bot] <167856002+mongodb-dbx-release-bot[bot]@users.noreply.github.com>

PYTHON-5639

Summary

Update version for 4.15.4 release.

Changes in this PR

Update package version.

Testing Plan

Standard tests

Screenshots (optional)

N/A

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?

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)

[sleepyStick]

LGTM!