Drop official support for Python 3.7

[ewels]

Python 3.7 hit EOL (end of life) in June: https://endoflife.date/python

I'm not aware of any 3.7-specific language constructs that we're specifically maintaining, but it might be nice to drop official support (CI) anyway to try to encourage users to update.

[ewels]

This needs updating in the CI, the docs, and also in the version check:
https://github.com/ewels/MultiQC/blob/f4878540c0b1e77fa419b589ec33f3a5039d17da/multiqc/multiqc.py#L477-L484

[vladsavelyev]

If we drop 3.7 support, it would simplify #2113 (we would be able to use shutil.copytree(..., dirs_exist_ok=True) introduced in 3.8).

However, we are still supporting even 3.6. I'd go with dropping both 3.6 and 3.7 in 1.17.

[ewels]

Done in #2121 but looks like we missed that multiqc.py mention..