Make parsing [testenv] sections in setup.cfg official

[mauvilsa]

Even though #1724 was reverted, I suppose that to make testenv sections in setup.cfg an official feature, there should be unit tests for it and be mentioned in the documentation. So I open this new pull request. I have modified the code so that it is backward compatible and added a unit test so that now it is tested for both [testenv] and [tox:testenv] sections.

Self-referencing #1727 since there is no issue associated to this.

Contribution checklist:

(also see CONTRIBUTING.rst for details)

• wrote descriptive pull request text
• added/updated test(s)
• updated/extended the documentation
• added relevant issue keyword
  in message body
• added news fragment in changelog folder
  • fragment name: <issue number>.<type>.rst for example (588.bugfix.rst)
  • <type> is must be one of bugfix, feature, deprecation,breaking, doc, misc
  • if PR has no issue: consider creating one first or change it to the PR number after creating the PR
  • "sign" fragment with "by :user:<your username>"
  • please use full sentences with correct case and punctuation, for example: "Fix issue with non-ascii contents in doctest text files - by :user:superuser."
  • also see examples
• added yourself to CONTRIBUTORS (preserving alphabetical order)

[gaborbernat]

[gaborbernat]

Perhaps the right wording here is officially/explicitly. Because the support is already there, just not spelled out.

[mauvilsa]

Support with the tox: prefix is not there yet.

[gaborbernat]

Ah no, sorry, miss-understood then. We're not planning on adding new ways to do this. You can already achieve what you want via [tox] and [testenv] which should suffice. I see no reason to support yet another way.

[mauvilsa]

@gaborbernat I guess you meant that we can already achieve this with [tox:tox] and [testenv]. Sure if this is what you want I can change the pull request only adding one unit test and the update of the documentation. Though I don't have time to do this right now.

[gaborbernat]