[MRG+1] Add SCRAPY_CHECK environment variable

[Matthijsy]

This PR implements the environment variable when running scrapy check as asked in #3704 . This way it is possible to have different behaviour in the scraper when running the check (like requiring less settings to be set)

fixes #3704

[codecov]

Codecov Report

Merging #3739 into master will increase coverage by <.01%.
The diff coverage is 54.54%.

@@            Coverage Diff             @@
##           master    #3739      +/-   ##
==========================================
+ Coverage   85.42%   85.43%   +<.01%     
==========================================
  Files         169      169              
  Lines        9635     9647      +12     
  Branches     1433     1436       +3     
==========================================
+ Hits         8231     8242      +11     
- Misses       1156     1157       +1     
  Partials      248      248

Impacted Files	Coverage Δ
scrapy/utils/misc.py	96.29% <100%> (+0.58%)	⬆️
scrapy/commands/check.py	25.35% <9.09%> (-0.37%)	⬇️

[Gallaecio]

Could you please add an automated test to cover this change?

[Matthijsy]

I've added a test for the set_environ util. I couldn't find the test of scrapy check so don't know how I should write a test for that part. Is this untested or can you tell me where I can find it?

[Gallaecio]

scrapy/tests/test_cmdline/__init__.py tests some commands, but there are no tests for scrapy check currently, no, so I’m OK with your test coverage.

It would be great if you could cover the new environment variable in the documentation about scrapy check. Other than that, I think this is ready to merge.

[Matthijsy]

@Gallaecio Thanks for your feedback! I've added the documentation, hope this is the right way to document it and otherwise I would love to update the documentation.

[Gallaecio]

Please, build the documentation locally to see how your changes look.

To build the documentation, you can create a virtual environment with the dependencies from requirements-py3.txt and docs/requirements.txt, and run make html from the docs folder. You can then open docs/build/html/index.html in a web browser and reach the contracts page.

[Matthijsy]

Thanks for the comment on how to build the documentation! I builded it and think it looks fine now

[Gallaecio]

I’ve refactored your documentation a bit. Please, review my changes.

[Matthijsy]

Great! Looks fine to me