Exclude docs/ from default pytest directories

[akihironitta]

What does this PR do?

Follow-up to #10012.

Problem

VS Code collects tests with:

$ pytest -- --rootdir ~/work/github.com/PyTorchLightning/pytorch-lightning -s --cache-clear

and it fails as the command unexpectedly runs docs/source/conf.py.

Does your PR introduce any breaking changes? If yes, please list them.

Before submitting

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• Did you read the contributor guideline, Pull Request section?
• Did you make sure your PR does only one thing, instead of bundling different changes together?
• [n/a] Did you make sure to update the documentation with your changes? (if necessary)
• [n/a] Did you write any new necessary tests? (not for typos and docs)
• [n/a] Did you verify new and existing tests pass locally with your changes?
• Did you list all the breaking changes introduced by this pull request?
• [n/a] Did you update the CHANGELOG? (not for typos, docs, test updates, or internal minor changes/refactorings)

PR review

Anyone in the community is welcome to review the PR.
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Did you have fun?

Make sure you had fun coding 🙃

Additional context

I'm moving from Emacs to VS Code and trying to setup my dev env :)

[Borda]

but we have some tests there, right?

[codecov]

Codecov Report

Merging #10078 (3294eca) into master (6609b2e) will increase coverage by 4%.
The diff coverage is n/a.

❗ Current head 3294eca differs from pull request most recent head d53a05a. Consider uploading reports for the commit d53a05a to get more accurate results

@@           Coverage Diff            @@
##           master   #10078    +/-   ##
========================================
+ Coverage      89%      92%    +4%     
========================================
  Files         182      182            
  Lines       16431    16115   -316     
========================================
+ Hits        14568    14898   +330     
+ Misses       1863     1217   -646     

[akihironitta]

I've just made sure locally that make doctest runs doctest in docs/* with the config updated in this PR, so make doctest is independent from setup.cfg changed here.