Add Poetry buildpack

[RobertRosca]

As suggested in the issue #835 this PR adds in a poetry buildpack.

This is heavily based on the implementation of the Pipenv buildpack, since they're pretty similar anyway. I've also implemented equivalent tests to what is done for pipenv.

The only large issue with this PR is how the python versioning is worked out in the buildpack. Poetry lets you specify dependency constraints using a variety of symbols (e.g. ^, *, ~), and it's likely that python was specified with these requirements.

To handle these constraints poetry-core has a semver module which is what I use to parse the versions constraints, however this means that poetry-core would then be a dependency requirement for repo2docker. I really don't like adding this dependency in, but the alternative is to either re-implement this constraint system, include a copy of the poetry.core.semver module in the poetry buildpack, or to ignore the constraints and parse them naively. Does anybody have a better way to deal with this?

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[RobertRosca]

Hi @betatim, just poking you in case the PR fell a bit too far down the todo list :stuck_out_tongue: no rush though! Oh and Happy New Year!

[RobertRosca]

Rebased onto main since there were some conflicts

[kylebarron]

it's likely that python was specified with these requirements.

Not just likely; specifying the Python version is mandatory. From here:

Be aware that declaring the python version for which your package is compatible is mandatory