Convert NextProtoNegotiationTests to use pytest-style tests (#569)

* Convert NextProtoNegotiationTests to use pytest-style tests

Addresses #340.

* Fix Twisted test suite

See https://twistedmatrix.com/trac/ticket/8876

* Remove tests for OpenSSL with no NPN support

NPN was added in OpenSSL 1.0.1.  As of version 16.1.0, PyOpenSSL
only supports OpenSSL 1.0.1 or later, so this test is:

1. Testing a use case that's no longer supported.
2. Dead code in CI.
2 files changed