Quiet deprecation warnings from imp module

The "imp" module is deprecated and should be replaced by "importlib". As
a consequence, loading the django settings used to raise deprecation
warnings:

    DeprecationWarning: the imp module is deprecated in favour of
    importlib; see the module's documentation for alternative uses

It should be noted that python 3.5.1 ships with an older release of
distutils which still relies on the imp module. Thus, users of python
3.5.1 (for instance: edx.org developers) will continue to see the
deprecation warning for some time, despite this patch. We suggest
upgrading to python 3.5.9.

This addresses part of CRI-196.

cms/envs/common.py

@@ -40,7 +40,7 @@ When refering to XBlocks, we use the entry-point name. For example,
 
 # pylint: disable=unused-import, useless-suppression, wrong-import-order, wrong-import-position
 
-import imp
+import importlib.util
 import os
 import sys
 from datetime import timedelta
@@ -1623,10 +1623,8 @@ OPTIONAL_APPS = (
 for app_name, insert_before in OPTIONAL_APPS:
     # First attempt to only find the module rather than actually importing it,
     # to avoid circular references - only try to import if it can't be found
-    # by find_module, which doesn't work with import hooks
-    try:
-        imp.find_module(app_name)
-    except ImportError:
+    # by find_spec, which doesn't work with import hooks
+    if importlib.util.find_spec(app_name) is None:
         try:
             __import__(app_name)
         except ImportError:

lms/envs/common.py

@@ -29,7 +29,7 @@ Longer TODO:
 # pylint: disable=invalid-name, wrong-import-position
 
 
-import imp
+import importlib.util
 import sys
 import os
 
@@ -3181,10 +3181,8 @@ OPTIONAL_APPS = [
 for app_name, insert_before in OPTIONAL_APPS:
     # First attempt to only find the module rather than actually importing it,
     # to avoid circular references - only try to import if it can't be found
-    # by find_module, which doesn't work with import hooks
-    try:
-        imp.find_module(app_name)
-    except ImportError:
+    # by find_spec, which doesn't work with import hooks
+    if importlib.util.find_spec(app_name) is None:
         try:
             __import__(app_name)
         except ImportError: