Update to the environment_{file|loader} feature to fix compatibility with python 3.{4,5,6}

Author: wynnw

supervisor/options.py

@@ -1978,8 +1978,11 @@ def load_external_environment_definition_for_config(cls, config):
             try:
                 from subprocess import check_output, CalledProcessError
                 kwargs = dict(shell=True)
-                if not PY2:
-                    kwargs['text'] = True
+                if sys.version_info.major >= 3:
+                    if sys.version_info.minor >= 7:
+                        kwargs['text'] = True
+                    else:
+                        kwargs['encoding'] = 'utf8'
 
                 envdata = check_output(config.environment_loader, **kwargs)