build: Add a distutils-based build system for the Python bits

This is a streamlined version of the libxml2 setup.py.in that is
adapted for libxslt so that one can build the Python bindings and build
wheels easily, especially for Windows builds.

This will bundle the libxslt and libexslt DLLs, which matters more for
Python 3.8.x and later on Windows, where the dependent DLLs need to be
found at where the .pyd files are located unless one uses
os.add_dll_directory(); the libxml2 (and its dependent DLLs) should be
located with the libxml2-python package.