Fail to run tests

[bunder]

Host information

``` $ cat /etc/os-release NAME="Arch Linux" PRETTY_NAME="Arch Linux" ID=arch BUILD_ID=rolling ANSI_COLOR="0;36" HOME_URL="https://www.archlinux.org/" DOCUMENTATION_URL="https://wiki.archlinux.org/" SUPPORT_URL="https://bbs.archlinux.org/" BUG_REPORT_URL="https://bugs.archlinux.org/" LOGO=archlinux $ python --version Python 3.8.0 $ pip list Package Version ----------------------------- ----------- absl-py 0.9.0 alabaster 0.7.12 anytree 2.7.3 apipkg 1.5 appdirs 1.4.3 arrow 0.15.2 asn1crypto 1.2.0 astor 0.8.0 attrs 19.3.0 Babel 2.7.0 bcrypt 3.1.7 Beaker 1.11.0 beautifulsoup4 4.8.1 binwalk 2.2.0 btrfsutil 1.1.1 CacheControl 0.12.5 cached-property 1.5.1 ceph-volume 1.0.0 cephfs 2.0.0 cephfs-shell 0.0.1 cffi 1.13.2 chardet 3.0.4 Click 7.0 colorama 0.4.3 coverage 4.5.4 cryptography 2.8 cycler 0.10.0 Cython 0.29.14 decorator 4.4.1 distlib 0.3.0 distro 1.4.0 docker 4.1.0 docker-compose 1.25.0 docker-pycreds 0.4.0 dockerpty 0.4.1 docopt 0.6.2 docutils 0.14 filelock 3.0.12 gast 0.2.2 grpcio 1.25.0 h5py 2.10.0 html5lib 1.0.1 httplib2 0.13.1 idna 2.8 imagesize 1.1.0 importlib-metadata 0.23 iniconfig 1.0.0 iotop 0.6 isc 2.0 jedi 0.15.1 Jinja2 2.10.3 jsonschema 3.2.0 Keras-Applications 1.0.8 Keras-Preprocessing 1.1.0 kiwisolver 1.1.0 lensfun 0.3.2 libvirt-python 5.8.0 lit 0.9.0.dev0 lockfile 0.12.2 louis 3.12.0 lxml 4.4.2 Mako 1.0.14 mallard-ducktype 1.0.2 Markdown 3.1.1 MarkupSafe 1.1.1 matplotlib 3.1.1 meld 3.20.1 meson 0.52.1 more-itertools 8.0.2 msgpack 0.6.2 namcap 3.2.10 nose 1.3.7 numpy 1.17.4 opt-einsum 3.1.0 ordered-set 3.1.1 packaging 19.2 paramiko 2.6.0 parso 0.5.1 pep517 0.6.0 Pillow 6.2.1 pip 19.2.3 pluggy 0.13.1 ply 3.11 progress 1.5 protobuf 3.11.1 psutil 5.6.7 pwquality 1.4.2 py 1.8.0 pyalpm 0.9.0 pyasn1 0.4.8 PyAudio 0.2.11 pycairo 1.18.2 pycparser 2.19 pyelftools 0.25 Pygments 2.5.2 PyGObject 3.34.0 PyNaCl 1.3.0 pyparsing 2.4.5 Pypubsub 4.0.3 PyQt5 5.13.2 PyQt5-sip 12.7.0 PyQtWebEngine 5.13.2 pyrsistent 0.15.6 python-dateutil 2.8.1 pytoml 0.1.21 pytz 2019.3 pyxdg 0.26 PyYAML 5.1.2 rados 2.0.0 rbd 2.0.0 requests 2.22.0 retrying 1.3.3 rgw 2.0.0 Send2Trash 1.5.0 setuptools 41.6.0 sh 1.12.14 sip 4.19.20 six 1.13.0 snowballstemmer 2.0.0 soupsieve 1.9.5 Sphinx 2.2.1 sphinx-rtd-theme 0.4.3 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 sphinxcontrib-websupport 1.1.2 team 1.0 tensorflow 2.1.0rc1 tensorflow-estimator 2.0.0 termcolor 1.1.0 texttable 1.6.2 toml 0.10.0 tox 3.14.2 tqdm 4.28.1 ujson 1.35 urllib3 1.25.7 virtualenv 16.1.0 webencodings 0.5.1 websocket-client 0.56.0 wheel 0.33.6 Whoosh 2.7.4 wrapt 1.11.2 wxPython 4.0.7.post2 youtube-dl 2019.11.28 zipp 0.6.0 ```

Running nosetests -xdv I get strange error:

test_functional.test_buildhtml_alt ... ERROR

======================================================================
ERROR: test_functional.test_buildhtml_alt
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/nose/case.py", line 197, in runTest
    self.test(*self.arg)
  File "/home/aleontiev/projects/sphinx/plantuml/tests/test_functional.py", line 58, in test
    runsphinx(src, builder, confoverrides)
  File "/home/aleontiev/projects/sphinx/plantuml/tests/test_functional.py", line 40, in runsphinx
    app = Sphinx(_srcdir, _fixturedir, _outdir, _outdir, builder,
  File "/usr/lib/python3.8/site-packages/sphinx/application.py", line 232, in __init__
    self.setup_extension(extension)
  File "/usr/lib/python3.8/site-packages/sphinx/application.py", line 395, in setup_extension
    self.registry.load_extension(self, extname)
  File "/usr/lib/python3.8/site-packages/sphinx/registry.py", line 478, in load_extension
    raise ExtensionError(__('Could not import extension %s') % extname, err)
sphinx.errors.ExtensionError: Could not import extension sphinx.builders.epub3 (exception: No module named 'sphinxcontrib.serializinghtml')
-------------------- >> begin captured stdout << ---------------------
Running Sphinx v2.2.1

--------------------- >> end captured stdout << ----------------------

----------------------------------------------------------------------
Ran 1 test in 0.370s

FAILED (errors=1)

sphinxcontrib-serializinghtml is installed and command python -c 'import sphinxcontrib.serializinghtml' executes successfully.

It fails with the same error in tox.