Move wsgiref.types to _typeshed.wsgi (#4175)

Re-export the types from wsgiref.types for now to avoid breaking
existing code. wsgiref.types should be removed eventually.

Also, reduce the boilerplate description in _typeshed/wsgi.pyi as it
mirrors the description in _typeshed/__init__.pyi.

tests/stubtest_whitelists/py3_common.txt

@@ -5,7 +5,7 @@ _importlib_modulespec
 _operator.methodcaller
 _threading_local.local.__new__
 _types
-_typeshed
+_typeshed.*  # Utility types for typeshed, doesn't exist at runtime
 _weakref.CallableProxyType.__getattr__
 _weakref.ProxyType.__getattr__
 _weakref.ReferenceType.__call__