str method start/end parameters should accept values supporting __index__

[gwk]

str count, find, index, rfind, and rindex all accept int, None, or any value implementing __index__ for the start/end parameters. PR #1109 improves the annotation from int to Optional[Int], but still neglects the __index__ protocol. Rather than proposing a typing.SupportsIndex ABC, @JelleZijlstra suggested that we wait until PEP 544 to deal with this.

Note that there is a similar problem with list and tuple, with the additional caveat that CPython 3.6 does not accept None values but the raised exception suggests that they are supposed to. This deserves a bugs.python.org report.

[gwk]

Filed http://bugs.python.org/issue29935 for the list/tuple concern.

[srittau]

Python 3.8 actually added SupportsIndex (which was backported to typing_extensions), so this should be possible now.

[asottile]

also noticing this for bytes.__getitem__ (and presumably __getitem__ for a lot of the builtin types as well)