Fix tests on 3.7 (#15543)

Author: JelleZijlstra

mypyc/irbuild/classdef.py

@@ -498,6 +498,9 @@ def populate_non_ext_bases(builder: IRBuilder, cdef: ClassDef) -> Value:
                 if builder.options.capi_version < (3, 8):
                     # TypedDict was added to typing in Python 3.8.
                     module = "typing_extensions"
+                    # It needs to be "_TypedDict" on typing_extensions 4.7.0+
+                    # and "TypedDict" otherwise.
+                    name = "_TypedDict"
             else:
                 # In Python 3.9 TypedDict is not a real type.
                 name = "_TypedDict"