deviceserver: replace use of imp (deprecated) with importlib.

The imp package is deprecated since Python 3.4.  Since we are now
dependent on at least Python 3.5, we can replace it with importlib.

In addition to stop using deprecated functionality, this change means
that reading the config no longer injects modules into sys.modules
(imp.load_source caused a microscope.config module to be added to
sys.modules).

Author: carandraug

microscope/deviceserver.py

@@ -24,7 +24,7 @@
 """
 
 from collections.abc import Iterable
-import imp # this has been deprecated, we should be using importlib
+import importlib.util
 import logging
 import multiprocessing
 import signal
@@ -352,8 +352,15 @@ def __main__():
         __console__()
 
 
+def _load_source(filepath):
+    spec = importlib.util.spec_from_file_location('config', filepath)
+    module = importlib.util.module_from_spec(spec)
+    spec.loader.exec_module(module)
+    return module
+
+
 def validate_devices(configfile):
-    config = imp.load_source('microscope.config', configfile)
+    config = _load_source(configfile)
     devices = getattr(config, 'DEVICES', None)
     if not devices:
         raise Exception("No 'DEVICES=...' in config file.")