[re] Add attribute '_pattern_type' to re. (#2877)

This is only available before python 3.7 based on:
https://github.com/python/cpython/blob/3.6/Lib/re.py#L283
https://github.com/python/cpython/blob/3.7/Lib/re.py

stdlib/3/re.pyi

@@ -71,6 +71,10 @@ else:
     TEMPLATE = 0
     _FlagsType = int
 
+if sys.version_info < (3, 7):
+    # undocumented
+    _pattern_type = ...  # type: type
+
 class error(Exception): ...
 
 @overload