A previous CL replaced "basestring" with "str", because basestring
does not exist in Python 3. However, this broke Python 2's ability
to interoperate with unicode strings. This CL introduces a workaround
(defining basestring to be equivalent to string, if it doesn't exist
already), and restores the references to basestring. This workaround
can be fixed when we're 100% on Python 3.

It also undoes some unnecessary and harder-to-read formatting changes.

Bug: 942522
Change-Id: I4a31ee46dc048134c2e4832b6c44ea00ce341899
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1572441
Commit-Queue: Aaron Gable <agable@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>