re: make _checkre2 a static method

There is no need to it to be a method, so we make it at least a static method to
make it easier to call.

mercurial/util.py

@@ -2211,7 +2211,8 b' except ImportError:'
 
 
 class _re:
-    def _checkre2(self):
+    @staticmethod
+    def _checkre2():
         global _re2
         global _re2_input
         if _re2 is not None: