Add support for Python 3.11

Add support for Python 3.12 and drop EOL 3.7

Analogous to gitpython-developers/gitdb#94,
this brings back Python 3.7 support (allowing it to be installed on
3.7, and testing on 3.7), even though 3.7 is end of life, because
support for 3.7 is not being dropped by GitPython (or gitdb) yet,
and there is value in keeping GitPython, gitdb, and smmap
consistent.

This reverts commit f1b4d14.

Revert "Drop support for EOL Python 3.7"

This updates smmap's CI configuration in ways that are in line with
recent updates to gitdb's. In most cases there is no difference in
the changes, and the reason for the updates is more to avoid
confusing differences than from the value of the changes
themselves. In one case, there is a major difference (fetch-depth).

- gitpython-developers/gitdb#89 (same)

- gitpython-developers/gitdb#90 (same)
  It's just the project, not dependencies, but otherwise the same.

- gitpython-developers/gitdb#92 (opposite)
  This is the major difference. We don't need more than the tip of
  the branch in these tests. Keeping the default fetch-depth of 1
  by not setting it explicitly avoids giving the impression that
  the tests here are doing something they are not (and also serves
  as a speed optimization).

- gitpython-developers/gitdb#93 (same)

Update CI, in line with gitdb

+ Make YAML indentation more consistent.

Since Python 3.12.0 stable has been released and is available via
setup-python, per:
https://github.com/actions/python-versions/blob/main/versions-manifest.json

This makes it so continue-on-error and allow-prereleases are no
longer special-cased to true for 3.12; instead, 3.12 is treaed the
same as other Python releases (where those are false).

No longer treat 3.12 as experimental on smmap CI