Move everything to Python 3.6 (#8835)

lib/python/qmk/cli/__init__.py

@@ -2,6 +2,8 @@
 
 We list each subcommand here explicitly because all the reliable ways of searching for modules are slow and delay startup.
 """
+import sys
+
 from milc import cli
 
 from . import cformat
@@ -19,5 +21,6 @@ from . import new
 from . import pyformat
 from . import pytest
 
-if not hasattr(cli, 'config_source'):
-    cli.log.warning("Your QMK CLI is out of date. Please upgrade with `pip3 install --upgrade qmk` or by using your package manager.")
+if sys.version_info[0] != 3 or sys.version_info[1] < 6:
+    cli.log.error('Your Python is too old! Please upgrade to Python 3.6 or later.')
+    exit(127)