Enhance `super_len` to count encoded bytes for str

[bruceadams]

This is a possible fix for issue #6586.

I am not at all confident that this is an appropriate change for Requests. All of the unit tests pass locally for me (specifically make ci).

[goelbenj]

@sigmavirus24 would you mind taking at these new changes?

[sigmavirus24]

closing and re-opening to trigger a new build