pythonPackages.pyftgl: fix build (#47232)

See https://hydra.nixos.org/build/80705583

Recent `boost` versions with `python` enabled have changed their naming
scheme for `boost_python` shared objects which causes issues with the
proper linking when building `pyftgl`.

Previously the library was named `boost_python3`, no it's named
`boost_python36` for current python (3.6.x).

The same issue applies for the `python2`.

Addresses #45960