Python 3 METH_VARARGS with METH_KEYWORDS

Added ```METH_VARARGS``` to the method definition flags in *_macosx.m* as Python 3 requires ```METH_VARARGS``` to be set when ```METH_KEYWORDS``` is set. This is already the case where ```METH_KEYWORDS``` is used elsewhere, namely *_ttconv.cpp* and *cntr.c*.

Without this and executing within Python3 one will get the error
```SystemError: Bad call flags in PyCFunction_Call. METH_OLDARGS is no longer supported!```