cros: start warning for python 3.6/3.7 usage

In preparation for updating chromite requirements to Python 3.8,
start warning when people run with older versions.

Bug: b:187794810
Change-Id: I7b5430384c0818bca05df5642d8583fd16204105
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4062907
Reviewed-by: Brian Ryner <bryner@google.com>
Commit-Queue: Brian Ryner <bryner@google.com>
Auto-Submit: Mike Frysinger <vapier@chromium.org>

cros

@@ -17,7 +17,7 @@ import sys
 
 
 # Min version of Python that we *want*.  We warn for older versions.
-MIN_PYTHON_VER_SOFT = (3, 6)
+MIN_PYTHON_VER_SOFT = (3, 8)
 # Min version of Python that we *require*.  We abort for older versions.
 MIN_PYTHON_VER_HARD = (3, 6)