fsmonitor: handle unicode keys in tuples
In Python 3, keys in the bset tuple are typically str, not
bytes. PyBytes_AsString() would return NULL. But we weren't
checking the return value and this would lead to a segfault.
This commit makes the code type and Python version aware. The
Python version specific code is to allow us to utilize a
modern API for converting str -> char* without having to
allocate an extra PyObject.
FWIW I wanted to assume that keys were always str. However,
there appear to be some bytes keys in some cases. I haven't
debugged this further.
Differential Revision: https://phab.mercurial-scm.org/D7210