tests: alphabetize and group Python imports & add check

- De-duplicates, groups, and sorts imports based on the provided `isort` settings.

tests/negtelnetserver.py

@@ -23,8 +23,7 @@
 #
 """A telnet server which negotiates."""
 
-from __future__ import (absolute_import, division, print_function,
-                        unicode_literals)
+from __future__ import absolute_import, division, print_function, unicode_literals
 
 import argparse
 import logging