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

test/test_response.py

@@ -1,8 +1,3 @@
-import pytest
-import capnp
-import os
-import time
-
 import test_response_capnp
 
 class FooServer(test_response_capnp.Foo.Server):