Port to python 3

Port code to python 3 yet remain backward compatible to python 2.
This port is in preparation for yocto 2.4 upgrade.

Partially resolves openbmc/openbmc#2920

Change-Id: I418395fc7fbda247d702551af69cbedbbdc3a7aa
Signed-off-by: CamVan Nguyen <ctnguyen@us.ibm.com>
diff --git a/servers/gevent/phosphor-gevent b/servers/gevent/phosphor-gevent
index 26cce22..5fa26fd 100644
--- a/servers/gevent/phosphor-gevent
+++ b/servers/gevent/phosphor-gevent
@@ -32,7 +32,7 @@
         sys.stderr.write('WSGI application required!')
         sys.exit(1)
 
-    exec 'from obmc.wsgi.apps.%s import App' % sys.argv[1]
+    exec('from obmc.wsgi.apps.%s import App' % sys.argv[1])
 
     default_cert = os.path.join(
         sys.prefix, 'share', os.path.basename(__file__), 'cert.pem')