be explicit about urllib import and remove conditional urllib(2) import

urllib and urllib2 have been in the python stdlib since at least
python-2.3.  There's no reason to conditionalize it.

Fixes https://github.com/ansible/ansible/issues/11322

lib/ansible/modules/extras/notification/flowdock.py

@@ -85,8 +85,7 @@ options:
     choices: ['yes', 'no']
     version_added: 1.5.1
 
-# informational: requirements for nodes
-requirements: [ urllib, urllib2 ]
+requirements: [ ]
 '''
 
 EXAMPLES = '''
@@ -104,6 +103,8 @@ EXAMPLES = '''
             tags=tag1,tag2,tag3
 '''
 
+import urllib
+
 # ===========================================
 # Module execution.
 #