Sphinx 2.0 problems: BuildEnvironment has no attribute warn_node

[hroncok]

In Fedora, we have the following problem when building the docs with Sphinx 2.0.1:

+ sphinx-build docs html
Running Sphinx v2.0.1
BUILDSTDERR: Configuration error:
BUILDSTDERR: There is a programmable error in your configuration file:
BUILDSTDERR: Traceback (most recent call last):
BUILDSTDERR:   File "/usr/lib/python3.7/site-packages/sphinx/config.py", line 361, in eval_config_file
BUILDSTDERR:     execfile_(filename, namespace)
BUILDSTDERR:   File "/usr/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 86, in execfile_
BUILDSTDERR:     exec(code, _globals)
BUILDSTDERR:   File "/builddir/build/BUILD/python-utils-2.3.0/docs/conf.py", line 71, in <module>
BUILDSTDERR:     original_warn_mode = sphinx.environment.BuildEnvironment.warn_node
BUILDSTDERR: AttributeError: type object 'BuildEnvironment' has no attribute 'warn_node'

I cannot have a look ATM, but can try to submit a PR later.

Reported at https://bugzilla.redhat.com/show_bug.cgi?id=1709063

[hroncok]

You apparently build docs by Python 2.7. That is essentially workarounding this, as it fetches Sphinx < 2. However when I adapt tox.ini like this:

diff --git a/tox.ini b/tox.ini
index 33b165c..7b51295 100644
--- a/tox.ini
+++ b/tox.ini
@@ -19,7 +19,7 @@ deps = flake8
commands = flake8 --ignore=W391 python_utils {posargs}

[testenv:docs]
-basepython = python2.7
+basepython = python3.7
whitelist_externals =
    rm
    cd

I can reproduce:

$ tox -e docs
GLOB sdist-make: .../python-utils/setup.py
docs inst-nodeps: .../python-utils/.tox/.tmp/package/1/python-utils-2.3.0.zip
docs installed: alabaster==0.7.12,Babel==2.6.0,certifi==2019.3.9,chardet==3.0.4,docutils==0.14,idna==2.8,imagesize==1.1.0,Jinja2==2.10.1,MarkupSafe==1.1.1,mock==3.0.5,packaging==19.0,Pygments==2.4.0,pyparsing==2.4.0,python-utils==2.3.0,pytz==2019.1,requests==2.21.0,six==1.12.0,snowballstemmer==1.2.1,Sphinx==2.0.1,sphinxcontrib-applehelp==1.0.1,sphinxcontrib-devhelp==1.0.1,sphinxcontrib-htmlhelp==1.0.2,sphinxcontrib-jsmath==1.0.1,sphinxcontrib-qthelp==1.0.2,sphinxcontrib-serializinghtml==1.1.3,urllib3==1.24.3
docs run-test-pre: PYTHONHASHSEED='4126083665'
docs runtests: commands[0] | rm -f docs/project_name.rst
docs runtests: commands[1] | rm -f docs/modules.rst
docs runtests: commands[2] | mkdir -p docs/_static
docs runtests: commands[3] | sphinx-apidoc -o docs/ python_utils
File docs/python_utils.rst already exists, skipping.
Creating file docs/modules.rst.
docs runtests: commands[4] | rm -f docs/modules.rst
docs runtests: commands[5] | sphinx-build -W -b html -d docs/_build/doctrees docs docs/_build/html
Running Sphinx v2.0.1

Configuration error:
There is a programmable error in your configuration file:

Traceback (most recent call last):
  File ".../python-utils/.tox/docs/lib/python3.7/site-packages/sphinx/config.py", line 361, in eval_config_file
    execfile_(filename, namespace)
  File ".../python-utils/.tox/docs/lib/python3.7/site-packages/sphinx/util/pycompat.py", line 86, in execfile_
    exec(code, _globals)
  File ".../python-utils/docs/conf.py", line 71, in <module>
    original_warn_mode = sphinx.environment.BuildEnvironment.warn_node
AttributeError: type object 'BuildEnvironment' has no attribute 'warn_node'

ERROR: InvocationError for command '.../python-utils/.tox/docs/bin/sphinx-build -W -b html -d docs/_build/doctrees docs docs/_build/html' (exited with code 2)
___________________________________ summary ____________________________________
ERROR:   docs: commands failed