[FIX] P3: remove pycompat.{keys,items,values} helpers

Now that we're closer to switching to P3 for good, these helpers have
outlived their usefulness, and mostly add noise.

All remaining dict.iter*() or dict.view*() must be converted to the
normal keys(), values() or items() calls.

Whenever the result is likely to be used for more than the scope of a
loop, or when the dict needs to be modified during iteration, the calls
must be wrapped in a ``list()``, to protect the new P3 semantics.
Those cases are very exceptional.

Also removed some dead code or improved the API to remove unnecessary
conversions.