prefixmaps should not restrict using Python 3.8

[pkalita-lbl]

I don't believe there's anything in the runtime code or dependencies of this package that preclude it from running on Python 3.8. And yet:

prefixmaps/pyproject.toml

Line 27 in 0bc0d5f

python = "^3.9"

I believe that change was introduced with #55, and specifically python = "^3.9" was to support sphinx = "^7.2.6". But given that sphinx is a development dependency, I'm not sure it's a good idea to be so aggressive about its version specification, especially when it influences the the minimum Python version.