This removes the logic that appended the git submodule directory
for smmap to sys.path under most circumstances when the version
of gitdb was not from PyPI. Now gitdb does not modify sys.path.

See gitpython-developers/GitPython#1717
and gitpython-developers/GitPython#1720 for
context. This change is roughly equivalent to the change to
GitPython, though as noted the behavior being eliminated is subtly
different here and there.

Changes worth mentioning:

- Format commands as code blocks instead of blockquotes.
  (This is particularly useful for the submodule update step, whose
  lines were inadvertently concatenated, but it also improves
  appearance overall.)

- Mention smmap as a requirement.
  (But also that it doesn't need to be separately installed.)

- Mention that gitdb-speedups is not currently maintained.

- No longer say gitdb has source code in gitorious.
  (Since that site no longer exists.)

- Call GitPython "GitPython" rather than "git-python".

- Replace the old git-python Google Groups link with a link to the
  Discussions page on the GitHub repository for GitPython.
  (This seems like the closest currently available resource.)

This changes it from monthly to weekly. See #99 and
gitpython-developers/GitPython#1702 (comment)
for context.

Never add a vendored smmap directory to sys.path

Revise and update the readme

Set Dependabot submodule update cadence to weekly

Add missing asserts in test_base.py

Use contextlib.suppress instead of except: pass

Add support for Python 3.13