use importlib instead of deprecated imp (#723)

On Python>=3.3.

Author: reginalin

pynamodb/_compat.py

@@ -1,8 +1,19 @@
 import six
+import sys
 
 if six.PY2:
     from inspect import getargspec as getfullargspec
 else:
     from inspect import getfullargspec
 
-__all__ = ('getfullargspec',)
+def load_module(name, path):
+    """Load module using the Python version compatible function."""
+    if sys.version_info >= (3, 3):
+        from imp import load_source
+        from importlib.machinery import SourceFileLoader
+        return SourceFileLoader(name, path).load_module()
+    else: 
+        from imp import load_source
+        return load_source(name, path)
+
+__all__ = ('getfullargspec', 'load_module')

pynamodb/settings.py

@@ -1,8 +1,8 @@
-import imp
 import logging
 import os
 import warnings
 from os import getenv
+from pynamodb._compat import load_module
 
 log = logging.getLogger(__name__)
 
@@ -20,7 +20,7 @@
 
 override_settings = {}
 if os.path.isfile(OVERRIDE_SETTINGS_PATH):
-    override_settings = imp.load_source('__pynamodb_override_settings__', OVERRIDE_SETTINGS_PATH)
+    override_settings = load_module('__pynamodb_override_settings__', OVERRIDE_SETTINGS_PATH)
     if hasattr(override_settings, 'session_cls') or hasattr(override_settings, 'request_timeout_seconds'):
         warnings.warn("The `session_cls` and `request_timeout_second` options are no longer supported")
     log.info('Override settings for pynamo available {}'.format(OVERRIDE_SETTINGS_PATH))