python.bzl should respect NEUROPOD_PYTHON_VERSION on Darwin

[vkuzmin-uber]

I am using Darwin, OSX Catalina that has default python 2 and also has python 3 (3.7.7).

I tried to build Neuropod with NEUROPOD_PYTHON_VERSION=3.7 and found that build failed at python_repo.

I found that this is because for Darwin python.bzl doesn't respect NEUROPOD_PYTHON_VERSION when it gets "libdir" and creates symlink then. As result it uses "python" and creats symlinks to python 2 even if NEUROPOD_PYTHON_VERSION=3.7 was requested.

I tested minor fix and it works after that. Will publish Pull Request.