Remove old urllib imports

All supported Python versions have the `urllib.parse` and
`urllib.request` modules.

voila/app.py

@@ -24,13 +24,8 @@
 
 from .voila_identity_provider import VoilaLoginHandler
 
-try:
-    from urllib.parse import urljoin
-    from urllib.request import pathname2url
-except ImportError:
-    from urllib import pathname2url
-
-    from urlparse import urljoin
+from urllib.parse import urljoin
+from urllib.request import pathname2url
 
 import jinja2
 import tornado.ioloop