[py] Fixed parsing of extended slices

[superbobry]

Prior to this commit the following failed to parse and as a result affected auto-indentation

for x in xs[i::j]: # <ENTER>

Interestingly, the new tests for slices pass with and without the grammar change because IIUC the variables are correctly tokenized regardless of the surrounding context.

[marijnh]

Thanks! Could you move the repeated expression into the preceding optional ((":" ExprNoComma? (":" ExprNoComma?)?)?) to avoid repeating the check when it doesn't match the first time? Also, please run npm run build-py and include the updated output in the commit.

[superbobry]

Good catch! Done.

I've also added a few more parsing fixes, PTAL.

[marijnh]

Wonderful. Merged!