Drop python 3.9

[sufikaur]

python 3.9 is end of life so it is not supported by new versions of requirements

[sufikaur]

Should other dependencies also be updated?

i.e.
"pint >= 0.24.1", # required to use Pyomo units. Pint 0.24.1 needed for Python 3.9 support

or
# sphinx-argparse 0.4.0 is the last version to support Python 3.9

[blnicho]

@sufikaur yes, I think it's worth checking if these other dependencies can be updated.