Implement Python version substitution per @Czaki

[psobolewskiPhD]

Description

Hi, here I'm adding python_version as a substitution, per @Czaki comment:
#1 (comment)
(and the followup).
To make it work, I needed to also add a substitution for python={python_version} and the conda env instruction.

You can check out the built docs in my fork:
https://github.com/psobolewskiPhD/napari-docs/actions/runs/3624911741

I also fixed the listing of supported python versions from 3.7 ... to 3.8 ... because we don't support 3.7 anymore.

Type of change

• Bug-fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

References

How has this been tested?

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• example: all tests pass with my change

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• My PR is the minimum possible work for the desired functionality
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• If I included new strings, I have used trans. to make them localizable.
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