Some of the modules imported by presubmit_support are unused, and some of those (cPickle, cStringIO) are not available on Python3, so remove them.
Also expose urllib2 via urllib_request and urllib_error to improve compatibility with Python3.

Bug: 1009814
Change-Id: I9f758df4270cc039df092e814c981a7a87b2c350
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1866170
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>