Properly support Python ver startup abort

Refactor ksconf.cli and ksconf.__main__ so that ksconf can explain that
you're Python is old and bad in a graceful way that doesn't require me
strip back out all the cool Python 3 stuff we've done so far.

This means that ksconf.__main__ modules should remain Python 2.7
(possibly 2.6, if module.__main__ even worked in that version??) *just*
so we can show a graceful error message if/when somebody tries to run
this on an old version of Python.

TL;DR: Trying to have our cake, and eat it too.

ksconf/__main__.py

@@ -0,0 +1,41 @@
+#!/usr/bin/env python
+# -*- coding: utf-8 -*-
+# PYTHON_ARGCOMPLETE_OK
+#
+# Unlike the rest of ksconf, this file should remain Python 2.7 compatible
+# This is just enough to gracefully report our presence in an unsupported
+# version of Python, if we happen to get installed there.  Try really hard
+# to avoid showing Python compiler errors as those are really confusing.
+
+""" KSCONF - Ksconf Splunk CONFig tool
+
+Optionally supports argcomplete for commandline argument (tab) completion.
+
+Install & register with:
+
+     pip install argcomplete
+     activate-global-python-argcomplete  (in ~/.bashrc)
+
+"""
+from __future__ import absolute_import, unicode_literals
+
+import sys
+
+if sys.version_info < (3, 7):   # noqa
+    from ksconf.consts import EXIT_CODE_BAD_PY_VERSION
+
+    # Can't actually import 'consts' because it loads 'enum'; so we cheat
+    EXIT_CODE_BAD_PY_VERSION = 121
+    sys.stderr.write(
+        "ABORT!  ksconf cannot run here because it requires Python 3.7 or later!\n\n"
+        "Check to see if you have multiple version of Python installed.\n"
+        "If you need Python 2.7 or 3.6 support, consider downgrading to an earlier release.\n"
+        "Releases 0.9.x support these older Python versions.\n\n"
+        "If you installed this with pip, try running:\n\n"
+        "   pip install -U 'kintyre-splunk-conf>=0.9,<=0.10'\n")
+    sys.exit(EXIT_CODE_BAD_PY_VERSION)
+
+from ksconf.cli import cli
+
+if __name__ == '__main__':  # pragma: no cover
+    cli()

ksconf/cli.py

@@ -20,18 +20,6 @@
 import sys
 from collections import defaultdict
 
-# Doh! This probably only works for Python 3.6; earlier version will fail due to f-string use (needs refactoring)
-if sys.version_info < (3, 7):   # noqa
-    from ksconf.consts import EXIT_CODE_BAD_PY_VERSION
-    sys.stderr.write(
-        "ERROR:  ksconf requires Python 3.7+.\n\n"
-        "If you need Python 2.7/3.6 support, consider downgrading.\n"
-        "Releases 0.9.x support these older Python versions.\n\n"
-        "You can try running:\n\n"
-        "   pip install -U 'kintyre-splunk-conf>=0.9,<=0.10'\n")
-    sys.exit(EXIT_CODE_BAD_PY_VERSION)
-
-
 import ksconf
 import ksconf.util
 from ksconf.commands import DescriptionHelpFormatterPreserveLayout, get_all_ksconf_cmds
@@ -221,8 +209,7 @@ def build_cli_parser(do_formatter=False):
 
 def cli(argv=None, _unittest=False):
     # TODO:  Rename '_unitest' to something more appropriate, maybe _exit=True?
-    if check_py is not None:
-        check_py()
+    check_py()
 
     parser = build_cli_parser(True)
     if not _unittest:
@@ -266,8 +253,4 @@ def check_py():
         if KSCONF_DEBUG not in os.environ:
             sys.exit(EXIT_CODE_ENV_BUSTED)
     # Okay, now NEVER call this code again....   (helpful for unit-testing & nested calls)
-    globals()["check_py"] = None
-
-
-if __name__ == '__main__':  # pragma: no cover
-    cli()
+    globals()["check_py"] = lambda: None