Use python3 in various python scripts

-B option is kept to avoid generating .pyc into the git repository.

Bug: 185116539
Test: ./update-tzdata.py
Test: ./download-iana-files.py --data
Test: debug_tools/host/visualize_zone_trees.py  -input input_data/android/countryzones.txt  -country_code us -output visualize_out
Test: debug_tools/host/dump-tzdata.py -tzdata output_data/iana/tzdata -output abc_out
Change-Id: I779e1dd74cbefcb6d911e09f43a5ccfe5effd6c3
diff --git a/update-tzdata.py b/update-tzdata.py
index c449e3a..6a05fb8 100755
--- a/update-tzdata.py
+++ b/update-tzdata.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python -B
+#!/usr/bin/python3 -B
 
 # Copyright 2017 The Android Open Source Project
 #