Drop support for Python 3.7

[soxofaan]

Python 3.7 reached end-of-life almost a year ago (2023-06-27)

This ticket is to collect reasons for dropping support for it.

[soxofaan]

PR #577 includes a sniff to skip STAC functionality on Python 3.7 and 3.8 (because it depends on pystac features that are not available there)

[soxofaan]

Python 3.8 officially reached end-of-life this month (https://devguide.python.org/versions/#unsupported-versions). I don't think we have to keep supporting two EOL'ed versions at this point

[soxofaan]

What dropping 3.7 will bring:

from https://nedbatchelder.com/text/which-py.html#h_38_october_14_2019_eol_october_2024:

• assignment expressions (walrus operator := )
• f-string “=” specifier
• positional-only parameters
• typing additions: TypedDict, Literal, Final, and Protocol
• new module: importlib.metadata

Full list of 3.8 changes.

Quite some interesting features (f-string “=” specifier, typing.Protocol and importlib.metadata) that could streamline development and maintenance

[soxofaan]

a silly unit test failure, only on python 3.7, for #652 fix was the last drop: removed python 3.7 from the github actions unittest workflow with 10d1b14

Still various todo's and cleanups left over, marked with #578, e.g. see 26bc871