Add support for PyTest Fixtures

[BenjamenMeyer]

PyTest is a nice testing framework that provides lots of integrations. As of PR #74 Stack-In-A-Box itself uses PyTest as the replacement for nosetests which is no longer supported. One feature of PyTest is Fixtures and it would be good to add fixture support to the utility modules at the bare minimum:

• PyTest Fixture for HTTpretty Util
• PyTest Fixture for Requests-Mock Util
• PyTest Fixture for Responses Util

The following may also be considered if possible:

• Generic PyTest Fixture