Implement slice object and string slicing

[chef-wally]

When using label_direction="RTL" the label is inverted, would it be possible to implement the below?

time.text = str(gatime - int(time() - gatimer))[::-1]
time.text = str(score)[::-1]

NotImplementedError: only slices with step=1 (aka none) are supported

[tannewt]

The error comes from here:

circuitpython/py/objstr.c

Line 455 in 4465adf

mp_raise_NotImplementedError(MP_ERROR_TEXT("only slices with step=1 (aka None) are supported"));

It doesn't look like there is a richer implementation that is turned off so this will take more work than just build tweaks.

[deshipu]

I think you could always do reversed(text).