Minimum supported/required Python version

[ptheywood]

Chaste's CMake currently requires Python >= 3.5.0, which is EOL so this could be increased.

Python EOL dates:

Python	EOL
3.5	2020-09-30
3.6	2021-12-23
3.7	2023-06-27
3.8	2024-10

https://devguide.python.org/versions/

Ubuntu 20.04 ships with Python 3.8, which may be a sensible new minimum.

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Suggested during #242