Don't fallback to PyDict_GetItemWithError() if the hash is unknown: compute the
hash instead. Add also comments to explain the optimization a little bit.

Issue #25472: In B[<type>], insert B in front of __bases__, to make the __dict__ descriptor work. (Merge 3.5->3.6)

See https://github.com/python/asyncio/pull/293 for details.

This change won't do anything in CPython 3.4

See https://github.com/python/asyncio/pull/293 for details.

Extract of patch by Ronald Oussoren.

See https://github.com/python/asyncio/pull/292 for details.

issue #24164.