Throw a readable error on too old Python

podman-compose v1.0.6 is the last to support Python3.6. When newer
podman-compose version is used with older Python, podman-compose gives
only a confusing error. This commit gives a clear message to use upgrade
Python version.
A descriptive error can not be thrown, as line "from __future__ imports"
must occur at the beginning of the file, but older Python (older than
Python3.7) does not recognize __future__ and throws an error
immediately.
Therefore, a comment is used to inform the user to update his Python
version.

Signed-off-by: Monika Kairaityte <[email protected]>

podman_compose.py

@@ -7,7 +7,7 @@
 # https://docs.docker.com/compose/django/
 # https://docs.docker.com/compose/wordpress/
 # TODO: podman pod logs --color -n -f pod_testlogs
-from __future__ import annotations
+from __future__ import annotations  # If you see an error here, use Python 3.7 or greater
 
 import argparse
 import asyncio.exceptions