Problem seems to be in Modules/_multiprocessing/semaphore.c 
line 549 where "__enter__" is defined as an alias for 
semlock_acquire, as is "acquire" a few lines above. However,
while "acquire" specifies METH_VARARGS | METH_KEYWORDS,
"__enter__" has only METH_VARARGS.

semlock_acquire at line 60 has the signature:

  SemLockObject *self, PyObject *args, PyObject *kwds

When it is called via __enter__ kwds aren't passed in
and it gets a NULL pointer.

Suggested patch attached, defining __enter__ in the same
way as acquire. Test included.