setup.py: Add selectors34 dependency for SlapPopen

setup.py

@@ -80,6 +80,7 @@ setup(name=name,
           'distro',
           'subprocess32; python_version<"3"',
           'enum34; python_version<"3"',
+          'selectors34; python_version<"3"', # used by SlapPopen (grid/utils)
           'ipaddress; python_version<"3"',  # used by whitelistfirewall
         ] + additional_install_requires,
       extras_require=extras_require,

slapos/grid/utils.py

@@ -33,7 +33,6 @@ import hashlib
 import os
 import pkg_resources
 import pwd
-import selectors
 import stat
 import sys
 import logging
@@ -41,8 +40,10 @@ import psutil
 import time
 
 if sys.version_info >= (3,):
+  import selectors
   import subprocess
 else:
+  import selectors34 as selectors
   import subprocess32 as subprocess