Update README.md for new version string

[st-walker]

in light of #234. we now support python 3.9+

[st-walker]

README.md is updated, setup.py now refuses installation when using e.g. Python 3.8

Obtaining file:///Users/stuartwalker/python3.8-ocelot/ocelot
  Installing build dependencies ... done
  Checking if build backend supports build_editable ... done
  Getting requirements to build editable ... done
  Installing backend dependencies ... done
  Preparing editable metadata (pyproject.toml) ... done
Requirement already satisfied: numpy in /Users/stuartwalker/Library/Python/3.8/lib/python/site-packages (from ocelot==24.3.0) (1.20.2)
Requirement already satisfied: scipy in /Users/stuartwalker/Library/Python/3.8/lib/python/site-packages (from ocelot==24.3.0) (1.6.2)
Requirement already satisfied: matplotlib in /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages (from ocelot==24.3.0) (3.7.5)
Requirement already satisfied: pandas in /Users/stuartwalker/Library/Python/3.8/lib/python/site-packages (from ocelot==24.3.0) (1.2.3)
Requirement already satisfied: h5py in /Library/Frameworks/Python.framework/Versions/3.8/lib/python3.8/site-packages (from ocelot==24.3.0) (3.11.0)
INFO: pip is looking at multiple versions of ocelot to determine which version is compatible with other requirements. This could take a while.
ERROR: Package 'ocelot' requires a different Python: 3.8.10 not in '>=3.9'