Remove deprecated license classifier

GitPython project metadata specify the project's license primarily
through the value of `license`, currently passed as an argument to
`setuptools.setup`, which holds the string `"BSD-3-Clause"`. This
is an SPDX license identifier readily understood by both humans and
machines.

The PyPI trove classifier "License :: OSI Approved :: BSD License"
has also been specified in `setup.py`. However, this is not ideal,
because:

1. It does not identify a specific license. There are multiple
   "BSD" licenses in use, with BSD-2-Clause and BSD-3-Clause both
   in very wide use.

2. It is no longer recommended to use a trove classifier to
   indicate a license. The use of license classifiers (even
   unambiguous ones) has been deprecated. See:

   - https://packaging.python.org/en/latest/specifications/core-metadata/#classifier-multiple-use
   - https://peps.python.org/pep-0639/#deprecate-license-classifiers

This commit removes the classifier. The license itself is of course
unchanged, as is the `license` value of `"BSD-3-Clause"`.

(An expected effect of this change is that, starting in the next
release of GitPython, PyPI may show "License: BSD-3-Clause" instead
of the current text "License: BSD License (BSD-3-Clause)".)

This change fixes a warning issued by a subprocess of `pip` when
installing the package. The warning, until this change, could be
observed by running `pip install . -v` or `pip install -e . -v` and
examining the verbose output, or by running `pip install .` or
`pip install -e .` with the `PYTHONWARNINGS` environment variable
set to `error`:

    SetuptoolsDeprecationWarning: License classifiers are deprecated.
      !!

              ********************************************************************************
              Please consider removing the following classifiers in favor of a SPDX license expression:

              License :: OSI Approved :: BSD License

              See https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license for details.
              ********************************************************************************

      !!

(In preceding commits, `test_installation` has been augmented to
set that environment variable, surfacing the error. This change
should allow that test to pass, unless it finds other problems.)

Author: EliahKagan

setup.py

@@ -95,7 +95,6 @@ def _stamp_version(filename: str) -> None:
         # "Development Status :: 7 - Inactive",
         "Environment :: Console",
         "Intended Audience :: Developers",
-        "License :: OSI Approved :: BSD License",
         "Operating System :: OS Independent",
         "Operating System :: POSIX",
         "Operating System :: Microsoft :: Windows",