refactor post stop command execution for Python 2 and 3 compatibility

Author: hamedsh

setup.py

@@ -95,7 +95,7 @@
     author="Chris McDonough",
     author_email="chrism@plope.com",
     packages=find_packages(),
-    install_requires=[],
+    install_requires=['subprocess32'] if py_version[0] == 2 else [],
     extras_require={
         'test': ['pytest', 'pytest-cov']
     },

supervisor/process.py

@@ -3,7 +3,10 @@
 import os
 import signal
 import shlex
-import subprocess
+try:
+    import subprocess32 as subprocess
+except ImportError:
+    import subprocess
 import time
 import traceback