Update requirements-dev.txt (as long as we have it)

Although it seems likely that the requirements-dev.txt file will be
removed when the project definition is made declarative (discussed
in gitpython-developers#1716 comments), if not before, for now it exists and might be
in use, so this updates it with tools that are currently used but
not listed in any extras or other requirements files:

- ruff: This has replaced flake8 and its plugins (gitpython-developers#1862) as well as
  black (gitpython-developers#1865). Currently there is no separate extra for tooling
  that is not part of unit testing, with some such tools listed in
  test-requirements.txt. The `ruff` package belongs here rather
  than there for now because it should not be installed just to run
  unit tests, since Ruff has to be built from source on some rarer
  platforms like Cygwin, which would take a long time and/or fail.

- shellcheck: The PyPI package for this is a convenience for
  installing it in projects that are already using pip (shellcheck
  is neither written in Python nor a tool to scan Python code). It
  installs pre-built binaries, which are not available for all
  platforms.

These packages remain listed:

- pytest-icdiff: This seems not to have been promoted to be in the
  test-requirements.txt file and the `test` extra because it does
  not work as well for diffs from tests that the pytest runner runs
  but that are written for the unittest framework.

- pytest-profiling [commented out]: I am not sure what the status
  of this is, perhaps it has just not been experimented with
  enough to know if it would be useful for profiling in GitPython.

This requirements-dev.txt file has a few limitations that suggest
it should be removed altogether sometime soon:

- It is not updated regularly.

- It is not always clear why something is there. Originally I
  believe it was for tools where the desire to use the tool was
  established but the tool did not yet work or worked but performed
  checks for which code had to be fixed. That purpose has drifted.

- It uses a different naming convention from the
  test-requirements.txt file in active use.

- It cannot be readily used to create an extra in the current
  project definition in setup.py because the simple parsing done
  there will not recognize the `-r` lines and will not skip the
  comments, and neither enhancement should be done in setup.py
  since that would move things farther away from a declarative
  project definition.

- It will naturally go away when the project definition is made
  declarative, since it will then be feasible to define as many
  extras as desired without proliferating separate requirements
  files (while still allowing their contents to be statically
  available to tools).

Since it may go away soon and is not regularly updated, I have kept
the explanations for why particular packages are there out of it.
But as long as it exists it may as well list the tools that really
are being used yet are not explicitly listed as dependencies.

requirements-dev.txt

@@ -1,7 +1,8 @@
 -r requirements.txt
 -r test-requirements.txt
 
-# libraries for additional local testing/linting - to be added to test-requirements.txt when all pass
-
+# For additional local testing/linting - to be added elsewhere eventually.
+ruff
+shellcheck
 pytest-icdiff
 # pytest-profiling