put back configuration file without py extension and deprecate it

this change put back (and fix it) support of configuration files without
ython extension and warn about its usage.

gunicorn/app/base.py

@@ -3,6 +3,7 @@
 # This file is part of gunicorn released under the MIT license.
 # See the NOTICE for more information.
 import importlib.util
+import importlib.machinery
 import os
 import sys
 import traceback
@@ -94,9 +95,17 @@ def get_config_from_filename(self, filename):
         if not os.path.exists(filename):
             raise RuntimeError("%r doesn't exist" % filename)
 
+        ext = os.path.splitext(filename)[1]
+
         try:
             module_name = '__config__'
-            spec = importlib.util.spec_from_file_location(module_name, filename)
+            if ext in [".py", ".pyc"]:
+                spec = importlib.util.spec_from_file_location(module_name, filename)
+            else:
+                msg = "configuration file should have a valid Python extension.\n"
+                util.warn(msg)
+                loader_ = importlib.machinery.SourceFileLoader(module_name, filename)
+                spec = importlib.util.spec_from_file_location(module_name, filename, loader=loader_)
             mod = importlib.util.module_from_spec(spec)
             sys.modules[module_name] = mod
             spec.loader.exec_module(mod)