Close the open file handle using context manager

Signed-off-by: Kevin Jing Qiu <kevin.qiu@points.com>

compose/config/environment.py

@@ -2,6 +2,7 @@ from __future__ import absolute_import
 from __future__ import unicode_literals
 
 import codecs
+import contextlib
 import logging
 import os
 
@@ -31,7 +32,8 @@ def env_vars_from_file(filename):
     elif not os.path.isfile(filename):
         raise ConfigurationError("%s is not a file." % (filename))
     env = {}
-    for line in codecs.open(filename, 'r', 'utf-8'):
+    with contextlib.closing(codecs.open(filename, 'r', 'utf-8')) as fileobj:
+        for line in fileobj:
             line = line.strip()
             if line and not line.startswith('#'):
                 k, v = split_env(line)