moonraker: Verify python executable version

Moonraker requires Python 3.7 or greater.

Signed-off-by:  Eric Callahan <arksine.code@gmail.com>

docs/installation.md

@@ -2,7 +2,9 @@
 
 This document provides a guide on how to install Moonraker on a Raspberry
 Pi running Raspian/Rasperry Pi OS.  Other SBCs and/or linux distributions
-may work, however they may need a custom install script.
+may work, however they may need a custom install script.  Moonraker
+requires Python 3.7 or greater, verify that your distribution's
+Python 3 packages meet this requirement.
 
 Klipper should be installed prior to installing Moonraker.  Please see
 [Klipper's Documention](https://github.com/KevinOConnor/klipper/blob/master/docs/Installation.md)

moonraker/moonraker.py

@@ -4,6 +4,7 @@
 #
 # This file may be distributed under the terms of the GNU GPLv3 license
 import argparse
+import sys
 import importlib
 import os
 import time
@@ -443,6 +444,13 @@ def main():
         '%(asctime)s [%(filename)s:%(funcName)s()] - %(message)s')
     file_hdlr.setFormatter(formatter)
 
+    if sys.version_info < (3, 7):
+        msg = "Moonraker requires Python 3.7 or above.  Detected Version: %s" \
+            % (sys.version)
+        logging.info(msg)
+        print(msg)
+        exit(1)
+
     # Start IOLoop and Server
     io_loop = IOLoop.current()
     try: