Fixing flake8 warnings and errors

flake8 . --count --exit-zero --max-complexity=10 --max-line-length=127 --statistics --exclude benchmark

Excluding the benchmark directory (due to protobuf generated files)
Also removing some Python2 specific code

examples/addressbook.py

@@ -1,6 +1,5 @@
 from __future__ import print_function
-import os
-import capnp
+import capnp # noqa: F401
 
 import addressbook_capnp