py3.7 fix

PyUnicode_AsUTF8() returns const char* in py3.7 instead of char*. Making
changes to reflect that which should also be safe in
other Python versions.

src/CXX/Python3/IndirectPythonInterface.hxx

@@ -178,7 +178,7 @@ PYCXX_EXPORT int &_Py_UnicodeFlag();
 PYCXX_EXPORT void _XINCREF( PyObject *op );
 PYCXX_EXPORT void _XDECREF( PyObject *op );
 
-PYCXX_EXPORT char *__Py_PackageContext();
+PYCXX_EXPORT const char *__Py_PackageContext();
 };
 
 #endif    // __CXX_INDIRECT_PYTHON_INTERFACE__HXX__