Disallow bare except clauses (W0702).
Restrict bare except clauses.

* All exceptions that are meant to be caught derive
  from Exception.
Merge pull request #764 from Renelvon/no_bare_except

No bare except clauses
Clean up transport.py
* Remove several unused functions from transport.py
* Remove an unused parameter in CryptoTransportLayer.join_network
  and in CryptoTransportLayer.get_crypto_peer
* Remove an unused import