add types.GenericAlias.__mro_entries__

[tungol]

I used Iterable[object] because it matches the call signature of resolve_bases, but it could just as easily be Any because the specific implementation here doesn't look at the argument and always returns a 1-tuple of the __origin__ attribute.

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

steam.py (https://github.com/Gobot1234/steam.py)
+ steam/trade.py:189: error: Argument 1 of "__mro_entries__" is incompatible with supertype "GenericAlias"; supertype defines the argument type as "Iterable[object]"  [override]
+ steam/trade.py:189: note: This violates the Liskov substitution principle
+ steam/trade.py:189: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides