Need to specify compatible version(s) of Python

[cemaccam]

Installing dependencies via pip install -r requirements.txt fails for h5py==3.1.0, when running Python 3.10, because it has a backend dependency on numpy==1.19.3, which supports Python 3.6 - 3.9.
I recommend we specify which version of Python is recommended, or which range of versions are supported.

Setting up a Miniconda environment based on python=3.9 allows dependencies to install as per requirements.txt, although markdown==3.3.5 gives a warning that this version has been yanked due to an invalid dependency, fixed in 3.3.6 (will create separate issue for this).

[cemaccam]

Seems from activate_this.py that the app expects Python 3.8.