Drop Python 3.7 at end of life (2023-06-27)

[Zac-HD]

Previously: #3174, #2615, #2218. EOL date via https://devguide.python.org/versions/

• Check git grep -nE "# pragma: no (branch|cover)|[pP]y(thon)? ?3|sys\.version" -- . ':!*changes.rst' to see all of the locations we've tagged for version compatibility reasons (and a few false-positives)
• Bump target versions for shed (upstream), and run for pyupgrade etc.
• Update the minimum supported version of Numpy to 1.17.3
• Check what's new in Python 3.8 to see about features, e.g. Annotated and positional-only arguments, that might not be explicitly tagged in Hypothesis.
  • positional-only arguments
  • f"{a=}"
  • .replace() method on code objects
  • importlib.metadata always available (check for other backports we can drop too)
  • several typing changes

(more checklist items tbd)

draft branch: master...Zac-HD:hypothesis:drop-py37