remove support for EOL python 3.7

[aleks-sch]

python 3.7 is EOL as-of June-2023

I propose removing 3.7, and will submit associated PR, from

• setup.py supported versions
• requirements.txt
• example conda env

according to this SO post we should then increment major version.

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handily, this will enable using one of my favourite 3.8+ features of f-string self-documenting expressions e.g. f'{var=}'

var = "foo"
print(f"some {var=}")

[mountainrambler]

Looking forward to PR. Now that Colab is 3.9 it makes sense to drop 3.7.