[bot_update] Print Python version.

Print the Python version during "bot_update". This is useful for
visually auditing the environment in which things are running.

BUG=None
TEST=None
R=iannucci@chromium.org

Change-Id: I92be8a888e6f1e5c17205665dff36378e641cd81
Reviewed-on: https://chromium-review.googlesource.com/636450Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Daniel Jacques <dnj@chromium.org>

recipes/recipe_modules/bot_update/resources/bot_update.py

@@ -1019,6 +1019,7 @@ def prepare(options, git_slns, active):
 
 
 def checkout(options, git_slns, specs, revisions, step_text, shallow):
+  print 'Using Python version: %s' % (sys.version,)
   print 'Checking git version...'
   ver = git('version').strip()
   print 'Using %s' % ver