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

Author: wynnw

supervisor/options.py

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