RF: make new InWheelCtx class for ret_self case

Seperate class seemed cleaner
BF: fix check for no platform tags in script

Huh, None instead of empty tuple for an append argument with no
instance.
NF: add unique_by_index function

Function to return unique elements in a sequence, but in the order in
which they occur.
BF: add tags for unique Python, C API combinations

Collect unique python, capi combinations to combine with tags to add,
rather than using all combinations and therefore duplicates.
RF: use helper function in addplats script test

Use `get_winfo` helper function in addplat scripts test.
RF: more verbose feedback about written wheels
RF: remove unused imports

pyflakes run