Read description field from setup files

[ento]

Pull Request Check List

No corresponding issue in the issue tracker.

• Added tests for changed code.
• Updated documentation for changed code.

I work with a project that has a local path dependency and noticed that the description of the local package would get added or removed from poetry.lock seemingly at random when a new dependency was added or otherwise got updated. The local path dependency uses setup.py for historical reasons, and I've identified at least one path in which the description field becomes empty, which is when its metadata gets extracted through static analysis.

Minimally reproducible example: https://github.com/ento/poetry/tree/mcve-setup-py-missing-description
Running poetry lock using the current master branch (f5cb9f0) produces a lock file with an empty description for the local path dependency like so:

[..]
[[package]]
name = "localpackage"
version = "0.0.1"
description = ""
[..]

This PR updates the SetupReader class to fetch the description field just like how it reads the package name field.

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