Fix build error with Python 3.10 build on Windows

The Windows build for Python 3.10 is giving us an error about ssize_t
being undefined, so this commit fixes the problem by replacing it with
Py_ssize_t, which is consistent with what we do elsewhere in the file.

python/google/protobuf/pyext/unknown_fields.cc

@@ -219,7 +219,7 @@ const UnknownField* GetUnknownField(PyUnknownFieldRef* self) {
                  "The parent message might be cleared.");
     return NULL;
   }
-  ssize_t total_size = fields->field_count();
+  Py_ssize_t total_size = fields->field_count();
   if (self->index >= total_size) {
     PyErr_Format(PyExc_ValueError,
                  "UnknownField does not exist. "