Python-opsramp

Refactor the Python packaging in line with current recommendations.
No functional changes but I am moving to a new minor version nonetheless
because this changes potentially affects installation of the package,
particularly on older systems.

Anything in the range 200-299 is a successful result and in particular POST operations often return http CREATED or NO_CONTENT if there is no need to communicate anything except that the POST succeeded.

This release adds "files" to the low-level POST method, using the same syntax as the Python requests module. Also uses this to add file_upload and training_model to First Response Policies.

V3 of this module formally drops support for Python 2 which went end of life at the end of 2019.

It's an incompatible version release so I am moving the major version forward and, both in honor of the occasion and for clarity, I'm jumping over V2 and straight on to V3 for Python 3. This release actively removes code whose only reason for existing was to support Python 2, so this new code base not only doesn't support Python 2, we know that some parts of it definitely won't run. I also updated setup.py to prevent installation on versions older than 3.6 which was chosen as a base because it was a watershed in support for f-strings and asyncio, both of which we may well use in upcoming releases of this module now that the door is open.

Use argparse to add a --debug argument to all of the samples, which enables Python debug logging so that urllib3 and other libraries that we use will display their debug output (for example the URLs they are accessing) as a learning aid for users of the samples, and python-opsramp in general.

No functional changes, just a warning that support for Python 2 is ending soon.