Remove 3.9 (no longer supported upstream)

See https://pypy.org/posts/2024/08/pypy-v7317-release.html, especially:

> The keen-eyed will have noticed no mention of Python version 3.9 in the releases above. Typically we will maintain only one version of Python3, but due to PyPy3.9 support on conda-forge we maintained multiple versions from the first release of PyPy3.10 in PyPy v7.3.12 (Dec 2022). Conda-forge is sunsetting its PyPy support, which means we can drop PyPy3.9. Since that was the major driver of benchmarks at https://speed.pypy.org, we revamped the site to showcase PyPy3.9, PyPy3.10, and various versions of cpython on the home page. For historical reasons, the "baseline" for comparison is still cpython 3.7.19.