Only respect ALL environments when not in parallel

[guahki]

fixes #2167

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:

    Fixed an issue with non-ascii contents in doctest text files -- by :user:`superuser`.

  • also see examples
• added yourself to CONTRIBUTORS (preserving alphabetical order)

[guahki]

As

1. this is my first contribution to this project and
2. I'm not sure about the correct way to fix this

this is kind of a draft/request PR. I will happily create appropriate changelog when you tell me, this is a sensible way of fixing the problem.

Thanks for the awesome tox tool!

[gaborbernat]

Can you please add changelog and test for this?

[guahki]

I added the changelog.

Unfortunately I do not have the time to write a test right now (as I would have to dig deep into this to understand how to even test this behavior of the subprocess invocation). I would be happy, if anybody can assist with this.

[gaborbernat]

This still needs a test, check around the test suite to find instances of this.

[guahki]

I had a look into it and it wasn't that hard indeed.
Thanks for your patience!