Fix CI for Python 3.9

[knelli2]

Proposed changes

Because our containers are Ubuntu 22.04 and the actions/setup-python@v5 action was pulling python3.9 built for 24.04, there was a GLIBC version mismatch. Here we just pull from deadsnakes instead.

Upgrade instructions

Code review checklist

• The code is documented and the documentation renders correctly. Run
  make doc to generate the documentation locally into BUILD_DIR/docs/html.
  Then open index.html.
• The code follows the stylistic and code quality guidelines listed in the
  code review guide.
• The PR lists upgrade instructions and is labeled bugfix or
  new feature if appropriate.

Further comments

[nilsvu]

this is already installed