3.9.9 was released out of schedule as a hotfix for an argparse regression in Python 3.9.8 which caused complex command-line tools to fail recognizing sub-commands properly. Details in BPO-45235 <https://bugs.python.org/issue45235>. There are only three
other bugfixes in this release compared to 3.9.8. See the changelog <https://docs.python.org/release/3.9.9/whatsnew/changelog.html> for details on what changed.
Upgrading to 3.9.9 is highly recommended if you’re running Python 3.9.8.
The next Python 3.9 maintenance release will be 3.9.10, currently scheduled for 2022-01-03.