Check PRs for whitespace

[brettcannon]

See https://hg.python.org/hooks/file/tip/checkwhitespace.py for the hg.python.org solution. Need to check both code and docs. Could either set up a bot to do the check on the diff directly or somehow use Travis and its TRAVIS_PULL_REQUEST environment variable to check whitespace only on the changing code.

[brettcannon]

Looks like there is a GitHub API for getting files involved in a PR. Probably can't query GitHub w/o authentication, else you run the risk of reaching their quota.

[brettcannon]

I wonder if there is something we can borrow from scripts/Tools/patchcheck.py for this? That way we can keep it entirely on Travis. Maybe add a --travis file to patchcheck that only does the whitespace checks?

Otherwise I'm thinking that to solve this we may want to provide an endpoint on Bedevere that returns the list of files in a PR as a JSON list of strings. We then have a script in Tools/scripts/ that will call Bedevere for the file list based on TRAVIS_PULL_REQUEST (and a secret to validate the caller), and then run the appropriate scripts much like https://hg.python.org/hooks/file/tip/checkwhitespace.py. Then the return value of the script will flag whether a PR has any whitespace that needs normalization.