Make sre's SubPattern objects accept slice objects like it already accepts

simple slices.

Lib/sre_parse.py

@@ -134,6 +134,8 @@ class SubPattern:
     def __delitem__(self, index):
         del self.data[index]
     def __getitem__(self, index):
+        if isinstance(index, slice):
+            return SubPattern(self.pattern, self.data[index])
         return self.data[index]
     def __setitem__(self, index, code):
         self.data[index] = code