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/calculator_server.py

@@ -2,8 +2,6 @@
 
 from __future__ import print_function
 import argparse
-import socket
-import random
 import capnp
 
 import calculator_capnp
@@ -136,5 +134,6 @@ def main():
     server = capnp.TwoPartyServer(address, bootstrap=CalculatorImpl())
     server.run_forever()
 
+
 if __name__ == '__main__':
     main()