config.py: top-level docstring, import order, minor edits

config.py

@@ -1,9 +1,17 @@
+"""
+Code dealing with the configuration of the program.
+
+On Windows this uses the Registry to store values in a flat manner.
+Linux uses a file, but for commonality it's still a flat data structure.
+"""
+
 import numbers
 import sys
 import warnings
 from os import getenv, makedirs, mkdir, pardir
-from os.path import expanduser, dirname, exists, isdir, join, normpath
+from os.path import dirname, expanduser, isdir, join, normpath
 from sys import platform
+
 import semantic_version
 
 from constants import applongname, appname