bpo-28556: Various updates to typing (#28) (#77)

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


(cherry picked from commit b692dc8)

Lib/test/mod_generics_cache.py

@@ -0,0 +1,14 @@
+"""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