Move and refactor remove_base to unresolve function in iri_resolver

[mielvds]

Please review and merge #207 first!

This PR

• moves remove_base(base, iri) to iri_resolver.py and renames it to unresolve(iri, base) (in correspondence with resolve())
• refactor function to use standard lib urlparse and urlunparse
• migrate references to remove_base((base, iri))
• add unittests for unresolve(iri, base) in tests/test_iri_parser.py

[davidlehn]

Needs a changelog entry. Could this be a breaking change?

[anatoly-scherbakov]

Should class-based or function-based pytest testcases be chosen as uniform practice for this project?

[mielvds]

I personally prefer class-based, but I'm fine with both. The only reason why the specs are function-based, is because I couldn't figure out how to make it work otherwise :) What do you think?

[anatoly-scherbakov]

I must confess I never saw pytest class based tests in prod code. Everyone just writes functions after unittest ceased to be de facto standard. But that's just my anecdotal evidence.

[mielvds]

I have, so I guess it's personal preference. I suggest we keep it for now, being

• one tests/test_*.py for every lib/*.py (with the exception of test_manifests.py ofc)
• one class per function that needs testing

... and having a bigger discussion on this some time later

[mielvds]

@davidlehn I've updated the changelog! I haven't seen any breaking changes, at least not what the spec tests are concerned. Other parts of the lib, not sure, don't know the lib well enough yet. The use of this function is pretty contained, so I think it's ok to see whether any regressions pop up and report them as new bug issues.