bpo-28556: Various updates to typing (#28)

 various updates from upstream python/typing repo:

- Added typing.Counter and typing.ChainMap generics
- More flexible typing.NamedTuple
- Improved generic ABC caching
- More tests
- Bugfixes
- Other updates

* Add Misc/NEWS entry

* Add issue number

Lib/test/mod_generics_cache.py

+"""Module for testing the behavior of generics across different modules."""
+
+from typing import TypeVar, Generic
+
+T = TypeVar('T')
+
+
+class A(Generic[T]):
+    pass
+
+
+class B(Generic[T]):
+    class A(Generic[T]):
+        pass

Misc/NEWS

@@ -227,6 +227,10 @@ Extension Modules
 Library
 -------
 
+- Issue #28556: Various updates to typing module: typing.Counter, typing.ChainMap,
+  improved ABC caching, etc. Original PRs by Jelle Zijlstra, Ivan Levkivskyi,
+  Manuel Krebber, and Łukasz Langa.
+
 - Issue #29100: Fix datetime.fromtimestamp() regression introduced in Python
   3.6.0: check minimum and maximum years.