Python on Windows uses `.pyd`, not `.so`

[junyer]

Although the pybind_extension() rule currently passes a .so-suffixed name to the cc_binary() rule, Python on Windows uses .pyd, not .so. The file can simply be copied like @morambro did here, for example, but we can – and should – do better. Ideally, without immediately breaking existing users, who have been specifying the file name in data forever.