Fix the ReadTheDocs build

[Gallaecio]

@kmike noticed that autodoc was not working, any API documentation extracted from comments was missing in master.

This started happening when we removed Scrapy dependencies from docs/requirements.txt. The build log issued warnings, but Read The Docs was not configured to fail the build in case of warnings. Travis CI would have failed, weren’t it because Tox automatically installs Scrapy from setup.py, which ReadTheDocs does not.

These changes add the dependencies back to the file, and make warnings cause build failures in ReadTheDocs.

You can see it working at https://scrapy-gallaecio.readthedocs.io/en/fix-readthedocs/topics/request-response.html

I also tried just enabling to fail on warning, and indeed the ReadTheDocs build failed after that.

[codecov]

Codecov Report

Merging #4373 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #4373   +/-   ##
=======================================
  Coverage   84.13%   84.13%           
=======================================
  Files         166      166           
  Lines        9967     9967           
  Branches     1482     1482           
=======================================
  Hits         8386     8386           
  Misses       1325     1325           
  Partials      256      256

[kmike]

Thanks @Gallaecio!