Thanks to Georg Brandl for the patch.

now is interned. This leads to more compact pickle data with protocol 4.

test.  call it TestVarAnnotations instead.

Use _PyCFunction_FastCallKeywords() in ceval.c: it allows to remove a lot of
code from ceval.c which was only used to call C functions.

This is now an official "protected" API that can be used to write
classes that are duck-type-compatible with Future without subclassing
it.  (For that purpose I also changed isinstance(result, Future) to
check for this attribute instead.)

Hopefully Amber Brown can use this to make Twisted.Deferred compatible
with asyncio.Future.

Tests and docs are TBD. (Also there are more isinstance() checks to fix.)