Export fails if package depends on poetry and tox

[bmw]

I'm not certain the cause of the problem here, but this plugin fails if poetry 1.3.1 and tox 3.27.1 are selected in poetry.lock. This currently happens right now even if you let Poetry pick the poetry and tox versions. For instance, with this simple pyproject.toml file, right now I see the following:

$ poetry lock
Updating dependencies
Resolving dependencies... (1.2s)
$ poetry export

Dependency walk failed at virtualenv (>=16.0.0,!=20.0.0,!=20.0.1,!=20.0.2,!=20.0.3,!=20.0.4,!=20.0.5,!=20.0.6,!=20.0.7)

Here's the created poetry.lock file.

This may be related to #118.

[bmw]

I think the problem here is poetry.lock has two versions of virtualenv specified.

[bmw]

This is still reproducible leaving the poetry and tox versions unrestrained with the current packages on PyPI. I think what's going on is two versions of virtualenv are being selected to satisfy poetry, but the exporter fails because it doesn't know which one to export. I think fixing this may require trying to parse the packages that depend on the virtualenv so that two versions can be included in the exported requirements files with different environment markers which sounds tricky to me.

[bmw]

I believe this is the same issue as #183 which has received much more attention. Closing this in favor of that.