Add some gc safety around _mysql__fetch_row (#348)

Users of gc.get_referrers() could cause a SystemError to be raised if this function is running in a different python thread.

MySQLdb/_mysql.c

@@ -1373,9 +1373,15 @@ _mysql_ResultObject_fetch_row(
     convert_row = row_converters[how];
     if (maxrows) {
         if (!(r = PyTuple_New(maxrows))) goto error;
-        rowsadded = _mysql__fetch_row(self, &r, skiprows, maxrows,
-                convert_row);
+
+        // see: https://docs.python.org/3/library/gc.html#gc.get_referrers
+        // This function can get a reference to the tuple r, and if that
+        // code is preempted while holding a ref to r, the _PyTuple_Resize
+        // will raise a SystemError because the ref count is 2.
+        PyObject_GC_UnTrack(r);
+        rowsadded = _mysql__fetch_row(self, &r, skiprows, maxrows, convert_row);
         if (rowsadded == -1) goto error;
+        PyObject_GC_Track(r);
     } else {
         if (self->use) {
             maxrows = 1000;