Drop Python 3.7 support

[sisp]

Python 3.7 reached EOL on June 27. Should we drop support? It would simplify the code base a bit, e.g.:

• We could remove the shutil.copytree backport.
• We could simplify the typing.{Literal,Protocol,get_args} imports which require typing-extensions for Python 3.7.
• We could simplify the cached_property imports which require backports for Python 3.7, and we could drop the backports dependency.
• We could simplify the importlib.metadata import and drop the importlib-metadata dependency.
• And perhaps more ...

Would this require a new major release? I think projects handle dropping Python version support differently, some consider it a breaking change, some don't.

[yajo]

Yes, of course we should do this.

Dependency changes are not breaking changes, according to semver.