Fix various common mispellings.

Greg Stark <stark@mit.edu>

Commit: e1623c3959aac707732d7a6ad09adfb5751763b3
Author: Greg Stark <stark@mit.edu>
Date: 2016-06-03T15:08:45Z
Releases: 9.6.0
Fix various common mispellings.

Mostly these are just comments but there are a few in documentation
and a handful in code and tests. Hopefully this doesn't cause too much
unnecessary pain for backpatching. I relented from some of the most
common like "thru" for that reason. The rest don't seem numerous
enough to cause problems.

Thanks to Kevin Lyda's tool https://pypi.python.org/pypi/misspellings

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