Try to fix size conversion on windows

PythonInterpreter/PythonInterpreter.cc

@@ -266,7 +266,7 @@ namespace pybind11 { namespace detail {
             if (!PyUnicode_Check(source))
               return false;
 
-            long int size;
+            Py_ssize_t size;
             char *ptr = PyUnicode_AsUTF8AndSize(source, &size);
 
             if (!ptr) {