os.path.commonprefix deprecated in Python 3.15 #364

...which caused an --enable-werror (i.e., PYTHONWARNINGS=error) build against
python3-3.15.0~rc3-2.fc45.x86_64 to fail with

> NSPR [5/5] install ...
> Exception: os.path.commonprefix() is deprecated. Use os.path.commonpath() for longest path prefix.
> Traceback (most recent call last):
>   File ".../engine/workdir/UnpackedTarball/gyp/pylib/gyp/common.py", line 25, in __call__
>     return self.cache[args]
>            ~~~~~~~~~~^^^^^^
> KeyError: ('.../engine/workdir/UnpackedTarball/nss/nss', '.../engine/workdir/UnpackedTarball/nss/nss')
>
> During handling of the above exception, another exception occurred:
>
> Traceback (most recent call last):
>   File ".../engine/workdir/UnpackedTarball/gyp/pylib/gyp/input.py", line 524, in CallLoadTargetBuildFile
>     result = LoadTargetBuildFile(
>         build_file_path,
>     ...<6 lines>...
>         False,
>     )
>   File ".../engine/workdir/UnpackedTarball/gyp/pylib/gyp/input.py", line 379, in LoadTargetBuildFile
>     d = gyp.common.RelativePath(depth, os.path.dirname(build_file_path))
>   File ".../engine/workdir/UnpackedTarball/gyp/pylib/gyp/common.py", line 27, in __call__
>     result = self.func(*args)
>   File ".../engine/workdir/UnpackedTarball/gyp/pylib/gyp/common.py", line 161, in RelativePath
>     prefix_len = len(os.path.commonprefix([path_split, relative_to_split]))
>                      ~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>   File "<frozen genericpath>", line 109, in commonprefix
> DeprecationWarning: os.path.commonprefix() is deprecated. Use os.path.commonpath() for longest path prefix.
>
> make[1]: *** [.../engine/external/nss/ExternalProject_nss.mk:129: .../engine/workdir/ExternalProject/nss/build] Error 1