pybind: replace deprecated license classifiers with SPDX expression

pybind: replace deprecated license classifiers with SPDX expression

Replace the deprecated "License :: OSI Approved :: GNU Lesser General
Public License v2 or later (LGPLv2+)" classifier with the SPDX license
expression "LGPL-2.0-or-later" in the license field.

Recent setuptools versions warn that license classifiers are deprecated
in favor of SPDX expressions in the license field. This change eliminates
the deprecation warning while maintaining the same licensing terms.

References:
- https://setuptools.pypa.io/en/stable/history.html#v77-0-0
- https://packaging.python.org/en/latest/guides/writing-pyproject-toml/#license
- https://spdx.org/licenses/LGPL-2.0-or-later.html

The warning looked like:
```
[1/1] Generating ../../../lib/cython_modules/lib.3/rados.cpython-313-x86_64-linux-gnu.so
/usr/lib/python3.13/site-packages/setuptools/dist.py:759: SetuptoolsDeprecationWarning: License classifiers are deprecated.
!!

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

        License :: OSI Approved :: GNU Lesser General Public License v2 or later (LGPLv2+)

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

!!
  self._finalize_license_expression()
```

Signed-off-by: Kefu Chai <tchaikov@gmail.com>