Make `verify` check both server and kernel environments

tests/venv/py2-with-kernel-requirements/verify

@@ -1,7 +1,19 @@
 #!/usr/bin/env python2
+import os
 import sys
 
 print(sys.version_info)
 assert sys.version_info[:2] == (2, 7)
 
 import numpy
+
+try:
+    import nbgitpuller
+except ImportError:
+    pass
+else:
+    raise Exception("'nbgitpuller' shouldn't have been installed from requirements3.txt")
+
+# Python 3 is the executable used for the notebook server, this should
+# have nbgitpuller installed
+os.system("python3 -c 'import nbgitpuller'")