Disable Travis docs job until a fix is found

[vstinner]

bpo-29527: The Travis docs job is still broken. Revert previous
change (add again -W option), and disable the job instead until a
better fix is found and things calm down after the migration to
GitHub.

The Travis "docs" job is still broken, even after the change removing the -W option:
https://travis-ci.org/python/cpython/jobs/200552343

It's too late to find a better fix. I suggest to disable the job just to fix Travis, so other jobs will be more useful, and take time later to fix this docs job.

[vstinner]

It's too late to find a better fix.

See the latest comment on the issue, "Currently there are (...) 6,245 warnings!":
http://bugs.python.org/issue29527#msg287586

I misunderstood that the "docs" job doesn't build the documentation, but only run a linter: it uses "sphinx-build -b linkcheck" which seems to exit with code 1 if there is at least one warning (if I understood correctly).

[zware]

The issue is that we're trying to be too strict to begin with; first let's make sure it can actually build, and later tighten it up. We should remove the -n (nit-picky mode), -b linkcheck (add linkcheck builder) and -W (warnings as errors) flags from SPHINXOPTS.

[codecov]

Codecov Report

Merging #14 into master will decrease coverage by -0.01%.

@@ Coverage Diff @@## master #14 +/- ## ========================================== - Coverage 82.37% 82.37% -0.01%  ========================================== Files 1427 1427 Lines 350948 350948 ========================================== - Hits 289092 289089 -3 - Misses 61856 61859 +3

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