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/thread_server.py

@@ -37,5 +37,6 @@ def main():
     server = capnp.TwoPartyServer(address, bootstrap=ExampleImpl())
     server.run_forever()
 
+
 if __name__ == '__main__':
     main()