bpo-29869: Allow underscores in numeric literals in lib2to3.

[nevsan]

I found this while debugging google/yapf#370. The fix is quite simple and quick.

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[nevsan]

I have completed the above steps.

[brettcannon]

Could you open an issue for this on bugs.python.org, @nevsan and mention it in the title? (see the contributing guidelines for info.)

[nevsan]

The existing regular expressions weren't actually strict enough as is, so I made them even more correct. In particular, we must have at least one digit following 0[xXbBoO] and must be before any underscores.

I have a small set of test cases to examine correctness of these regular expressions: https://gist.github.com/nevsan/7fc78dc61d309842406d67d6839b9861

[nevsan]

Updated regular expressions to avoid catastrophic backtracking. Gist updated with new test cases (still passes).

[brettcannon]

@nevsan looks like Travis disagrees that the tests still pass 😉

[nevsan]

Took a few iterations, but I managed to fix up the regexes. I also added a test case to test numeric literals with underscores (and verified that it fails for the un-patched HEAD).

[rhettinger]

LGTM

[brettcannon]

@rhettinger would you mind officially registering your review approval? You can do that by going under "Files changed", clicking the green "Review changes", and then you can choose "Approve" and submit the review. Perk of this is your approval will show up under Reviewers so if another core dev has the time to merge and backport they know someone else already cleared the code.

[Mariatta]

@nevsan Can you create the PR against the master branch?