basestring doesn't exist in Python 3. scm.Capture status doesn't need to support strings either.

Bug: 1002209
Change-Id: Ie9edfdfd09b789bfaace038a6ad8f3a7da344bc4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1890925Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>