Doc: -W flag for sphinx-build can be disabled

[JulienPalard]

Since 859c068, travis-ci is using -W via SPHINXOPTS so
we do not need it unconditionally in ALLSPHINXOPTS.

So I'm adding -W to Azure CI and droping it from ALLSPHINXOPTS.

Making -W optional would help with i18n builds (some versions, like
dev, are not actively maintained by translators, and can easily error, some countries are also less strict on sphinx errors and yields tons of thems).

cc @matrixise, @tirkarthi

[tirkarthi]

The reason over the change was to catch warnings as errors locally. If it's caught during CI I am okay. It's just that it causes a round trip where make docs passes locally and fails in CI but helps for translated builds.

I have added a NEWS entry in my previous PR which can be deleted as part of this PR?

https://devguide.python.org/committing/#what-s-new-and-news-entries

If a change is reverted prior to release, then the corresponding entry is simply removed. Otherwise, a new entry must be added noting that the change has been reverted (e.g. when a feature is released in an alpha and then cut prior to the first beta).

Ref : https://github.com/python/cpython/pull/8248/files#diff-07576315d4856b4c89dcc90f4a2704e2

[tirkarthi]

Approving the changes. News entry can be deleted to avoid confusion. Thanks :)

https://github.com/python/cpython/pull/8248/files#diff-07576315d4856b4c89dcc90f4a2704e2

[JulienPalard]

Implemented nice idea from @matrixise, @tirkarthi what do you think?

[tirkarthi]

@JulienPalard I am little confused here since my Makefile knowledge is low but isn't it the same as using -W directly and we now just use SPHINXERRORHANDLING that refers to -W? Users who want to disable this set SPHINXERRORHANDLING environment variable to ''?

[JulienPalard]

Users who want to disable this set SPHINXERRORHANDLING environment variable to ''?

Almost, the precedence in Makefile is "command line beats makefile variables beats environment", so we'll have to pass it as command line, like:

make html SPHINXERRORHANDLING=''

Which is nice as we could also pass:

make html SPHINXERRORHANDLING='-W --keep-going'

in the CI to get more than a single error in the report.

[tirkarthi]

@JulienPalard Thanks for the explanation. LGTM 👍

[matrixise]

LGTM for me! You can merge it.