pre-upload: start warning about older Python 3 versions

Let people know we're going to be dropping support so they can
migrate before we actually cut them off.

Bug: None
Test: unittests pass
Change-Id: I74734b43341b961a63a1b0ca96c13dbcad59d250
diff --git a/pre-upload.py b/pre-upload.py
index febbfa5..53b5ffb 100755
--- a/pre-upload.py
+++ b/pre-upload.py
@@ -38,6 +38,10 @@
     # We don't actually test <Python-3.6.  Hope for the best!
     print('repohooks: error: Python-3.4+ is required', file=sys.stderr)
     sys.exit(1)
+elif sys.version_info.major == 3 and sys.version_info < (3, 6):
+    # We want to get people off of old versions of Python.
+    print('repohooks: warning: Python-3.6+ is going to be required; '
+          'please upgrade soon to maintain support.', file=sys.stderr)
 
 
 _path = os.path.dirname(os.path.realpath(__file__))