Use python3 as default python command

After Python 2 is getting unsupported, new distros
like CentOS 8 and RHEL8 have stopped providing 'python'
package forcing user to decide which alternative to
use by installing 'python2' or 'python3.x' package
and then setting python alternative.

This change is intended to make using python3 command as
much as possible and use it as default 'python' alternative
where needed.

The final goals motivating this change are:
 - stop using python2 as much as possible
 - help adding support for CentOS 8 and RHEL8

Change-Id: I1e90db987c0bfa6206c211e066be03ea8738ad3f
diff --git a/tests/test_worlddump.sh b/tests/test_worlddump.sh
index f407d40..9196525 100755
--- a/tests/test_worlddump.sh
+++ b/tests/test_worlddump.sh
@@ -8,7 +8,7 @@
 
 OUT_DIR=$(mktemp -d)
 
-$TOP/tools/worlddump.py -d $OUT_DIR
+${PYTHON} $TOP/tools/worlddump.py -d $OUT_DIR
 
 if [[ $? -ne 0 ]]; then
     fail "worlddump failed"