Drop support for Python 3.7 and 3.8

[hugovk]

Python 3.8 is end-of-life next month:

• https://devguide.python.org/versions/
• https://peps.python.org/pep-0569/#lifespan

Many of the files under pyperformance/data-files/benchmarks/ have requires-python = ">=3.7" or requires-python = ">=3.8" but I've not changed them.

[mdboom]

LGTM.

I agree with your policy of not changing the minimum versions on benchmarks, unless we change their content which should be exceedingly rare.