add clarification for Python 3

Bug: None
Change-Id: Ie9cfc90b1dd4b241238e5f0a4c995979f7a0fbdc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2213175
Auto-Submit: Andreas Papacharalampous <andreas@apap04.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

README.md

 # depot_tools
 
-Tools for working with Chromium development. It requires python 2.7.
+Tools for working with Chromium development. It requires python 2.7 or 3.8 for python 3 support.
 
 
 ## Tools