Restricted galaxy-importer script to Python 3.6+

Ansible Galaxy content importer is using format strings [1] which
are supported since Python 3.6. Our Zuul CI job for OpenStack Queens
uses Ubuntu 16.04 LTS (Xenial Xerus) as its base image which has
Python 3.5 only.

Ref.:
[1] b7140d6b3b/galaxy_importer/main.py (L117)

Change-Id: I5d3b2f71937a0e4ab9a8d49df10744f7d95a7de2
(cherry picked from commit 9f60f0f26d)

tools/check-import.sh

@@ -15,5 +15,10 @@
 
 set -e
 
+if python -c 'import sys; sys.exit(0 if sys.version_info[0:2] < (3, 6) else 1)'; then
+    echo "Skipped Ansible Galaxy content importer check because it requires Python 3.6 or later" 2>&1
+    exit
+fi
+
 TOXDIR="${1:-.}"
 python -m galaxy_importer.main "$TOXDIR/build_artifact/"*