F20: Loosen restrictions on python3 versions for creating docker images

The scripts use for creating docker images for test exec server hardcode
in the usage of Python 3.6.

Update the scripts to allow >= Python 3.6 for usage.

BUG=None
TEST=Tested caller script with python2, python 3.7, 3.8, 3.9

Change-Id: I34b9c6ab65c495c22a2884772391f81d9f91c8ed
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/dev-util/+/3098520
Tested-by: Jesse McGuire <jessemcguire@google.com>
Tested-by: Derek Beckett <dbeckett@chromium.org>
Reviewed-by: Derek Beckett <dbeckett@chromium.org>
Commit-Queue: Jesse McGuire <jessemcguire@google.com>
diff --git a/test/container/utils/container_prep.py b/test/container/utils/container_prep.py
index 8788f5b..5a0e8a1 100644
--- a/test/container/utils/container_prep.py
+++ b/test/container/utils/container_prep.py
@@ -144,6 +144,11 @@
 
 def main():
     """Entry point."""
+
+    if not (sys.version_info.major == 3 and sys.version_info.minor >= 6):
+        print('python3.6 or greater is required.')
+        sys.exit(1)
+
     args = parse_local_arguments()
     builder = DockerPrep(args)
     builder.config_paths()
diff --git a/test/container/utils/container_prep_caller.sh b/test/container/utils/container_prep_caller.sh
index e88d2d8..08f1780 100644
--- a/test/container/utils/container_prep_caller.sh
+++ b/test/container/utils/container_prep_caller.sh
@@ -29,4 +29,4 @@
 create_venv
 
 # $chroot_path $sysroot_path $output_path are set as ENV VAR's.
-python3.6 container_prep.py -chroot=${chroot_path} -sysroot=${sysroot_path} -path=${output_path} -force_path=True -src=${src_root}
+python3 container_prep.py -chroot=${chroot_path} -sysroot=${sysroot_path} -path=${output_path} -force_path=True -src=${src_root}