'parse_requirements' does not consider environment markers

[hexagonrecursion]

Trying to parse the following file

pytest>=4.6.10,<5 ; python_version < '3'
pytest>=5.4.2,<6 ; python_version >= '3'

Throws an exception

PipError: Double requirement given: pytest<6,>=5.4.2 (already in pytest<5,>=4.6.10, name='pytest')

[di]

This is because parse_requirements is not considering environment markers. This library would need go do something similar to pip here: https://github.com/pypa/pip/blob/faee60baea2b850427b5fceb22383883fa11290b/src/pip/_internal/req/req_set.py#L90-L95

[woodruffw]

cc @tetsuo-cpp could you take a look at this? I think the Requirements class gives us everything we need, we just need to dedupe the requirements based on which one(s) satisfy the environment marker(s):

pip-api/pip_api/_vendor/packaging/requirements.py

Line 82 in 98d3aa8

class Requirement(object):

Edit: this PEP seems to exhaustively list all of the markers we need to support and their basic expression language: https://www.python.org/dev/peps/pep-0496/. But perhaps there's a (vendored) dependency we could reuse here.

Edit 2: Looks like we already have a marker evaluator vendored under packaging.markers, so we should probably use that!

[di]

Yes, packaging.markers should be sufficient here (and also what pip uses).

[tetsuo-cpp]

I think the code to use the marker is relatively straight forward (we just check marker.evaluate() prior to inserting the requirement into the dict). The problem I'm seeing is that the marker isn't being parsed properly and is being set to None when we construct the packaging.Requirement.

I'm looking at the parsing code to figure out what's going wrong here.