Instead add a comment about why this is really there,
and invoke sphinx python modules with python3 directly.

Some of the warnings from sphinx-build are really just warnings,
but a lot of the things we want to avoid and really break documentation,
like broken internal references and some invalid rst directives should
really be errors.

Now we treat all warnings as errors, this should ensure that
any commits landing upstream never break the docs.

This makes writes done by `bst track` atomic, meaning a crash or power
loss while overwriting a .bst file should never cause the the contents
of that file to be lost or partially written.

This avoids a circular dependency between the 'utils' and '_yaml'
modules.

This is a context manager that can be used to divert file writes into a
temporary file, which is then renamed into place once writing is
complete. It is primarily intended for use by source plugins which
download files, so they can ensure that their downloads appear atomic
and there is no risk of leaving half-downloaded files in the cache.

So far this is not used in the core, but it is needed by the Docker
source plugin that is proposed for the bst-external plugins repo. See:
https://gitlab.com/BuildStream/bst-external/merge_requests/9

Fixes the warning:

   WARNING: /path/to/buildstream/doc/source/install.rst:45: (WARNING/2) Title underline too short.

Recently I added the `reason` member which can be used to set
machine readable error reason strings for the purpose of testing.

Forgot to add the necessary `*` argument, forcing `reason` to be
a keyword-only argument.