FTBFS / error: Unmet build dependencies: python3-sphinxcontrib.spelling

1) apt-src install python3-coverage

result:

...
dpkg-source: info: applying 04.sphinx-add-code-path.patch
dpkg-checkbuilddeps: error: Unmet build dependencies: python3-sphinxcontrib.spelling
...

2) this seems to lead to a FTBFS

Running Sphinx v1.6.5
making output directory...

Exception occurred:
  File "conf.py", line 107, in <module>
ImportError: No module named sphinx_rtd_theme
The full traceback has been saved in /tmp/sphinx-err-ZzQfy4.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!
debian/rules:159: recipe for target 'override_dh_auto_build' failed
make[1]: *** [override_dh_auto_build] Error 1
make[1]: Leaving directory '/home/z/python/python3-coverage/python-coverage-4.4.1+dfsg.1'
debian/rules:71: recipe for target 'build' failed
make: *** [build] Error 2
dpkg-buildpackage: error: debian/rules build subprocess returned exit status 2
debuild: fatal error at line 1152:
dpkg-buildpackage -rfakeroot -us -uc -ui -b failed
z@aisha:~/python/python3-coverage/python-coverage-4.4.1+dfsg.1$ cat /tmp/sphinx-err-ZzQfy4.log

# Sphinx version: 1.6.5
# Python version: 2.7.14+ (CPython)
# Docutils version: 0.14
# Jinja2 version: 2.10
# Last messages:

# Loaded extensions:
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/sphinx/cmdline.py", line 305, in main
    opts.warningiserror, opts.tags, opts.verbosity, opts.jobs)
  File "/usr/lib/python2.7/dist-packages/sphinx/application.py", line 168, in __init__
    confoverrides or {}, self.tags)
  File "/usr/lib/python2.7/dist-packages/sphinx/config.py", line 150, in __init__
    execfile_(filename, config)
  File "/usr/lib/python2.7/dist-packages/sphinx/util/pycompat.py", line 150, in execfile_
    exec_(code, _globals)
  File "/usr/lib/python2.7/dist-packages/six.py", line 709, in exec_
    exec("""exec _code_ in _globs_, _locs_""")
  File "<string>", line 1, in <module>
  File "conf.py", line 107, in <module>
ImportError: No module named sphinx_rtd_theme

ubuntu: 18.04 bionic
python3-coverage: 4.4.1+dfsg.1-1
sphinx-doc:
  Installed: (none)
  Candidate: 1.6.5-3

That's on Ubuntu Bionic. There's no warning on apt-src install on Ubuntu Artful, but the end resulting FTBFS is the same:

# Sphinx version: 1.5.6
# Python version: 2.7.14 (CPython)
# Docutils version: 0.14
# Jinja2 version: 2.9.6
# Last messages:

# Loaded extensions:
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/sphinx/cmdline.py", line 295, in main
    opts.warningiserror, opts.tags, opts.verbosity, opts.jobs)
  File "/usr/lib/python2.7/dist-packages/sphinx/application.py", line 163, in __init__
    confoverrides or {}, self.tags)
  File "/usr/lib/python2.7/dist-packages/sphinx/config.py", line 134, in __init__
    execfile_(filename, config)
  File "/usr/lib/python2.7/dist-packages/sphinx/util/pycompat.py", line 129, in execfile_
    exec_(code, _globals)
  File "/usr/lib/python2.7/dist-packages/six.py", line 699, in exec_
    exec("""exec _code_ in _globs_, _locs_""")
  File "<string>", line 1, in <module>
  File "conf.py", line 107, in <module>
ImportError: No module named sphinx_rtd_theme

themusicgod1@eva1:~/python/python-coverage-4.3.4+dfsg.1$ apt-cache policy python3-coverage
python3-coverage:
  Installed: (none)
  Candidate: 4.3.4+dfsg.1-1build1
  Version table:
     4.3.4+dfsg.1-1build1 500
        500 https://gpl.savoirfairelinux.net/pub/mirrors/ubuntu artful/universe amd64 Packages
        500 https://mirror.its.sfu.ca/mirror/ubuntu artful/universe amd64 Packages
themusicgod1@eva1:~/python/python-coverage-4.3.4+dfsg.1$ apt-cache policy sphinx-doc
sphinx-doc:
  Installed: (none)
  Candidate: 1.5.6-2
  Version table:
     1.5.6-2 500
        500 https://gpl.savoirfairelinux.net/pub/mirrors/ubuntu artful/main amd64 Packages
        500 https://gpl.savoirfairelinux.net/pub/mirrors/ubuntu artful/main i386 Packages
        500 https://mirror.its.sfu.ca/mirror/ubuntu artful/main amd64 Packages
        500 https://mirror.its.sfu.ca/mirror/ubuntu artful/main i386 Packages
themusicgod1@eva1:~/python/python-coverage-4.3.4+dfsg.1$ apt-cache policy python3-sphinxcontrib.spelling
python3-sphinxcontrib.spelling:
  Installed: 2.3.0-1
  Candidate: 2.3.0-1
  Version table:
 *** 2.3.0-1 500
        500 https://gpl.savoirfairelinux.net/pub/mirrors/ubuntu artful/universe amd64 Packages
        500 https://gpl.savoirfairelinux.net/pub/mirrors/ubuntu artful/universe i386 Packages
        500 https://mirror.its.sfu.ca/mirror/ubuntu artful/universe amd64 Packages
        500 https://mirror.its.sfu.ca/mirror/ubuntu artful/universe i386 Packages
        100 /var/lib/dpkg/status

For what it's worth this happens in debian stable, too(reportbug'ing in 5...4..3...

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: python3-coverage 4.4.1+dfsg.1-1
ProcVersionSignature: Ubuntu 4.13.0-25.29-generic 4.13.13
Uname: Linux 4.13.0-25-generic x86_64
ApportVersion: 2.20.8-0ubuntu6
Architecture: amd64
Date: Fri Jan 19 17:53:41 2018
InstallationDate: Installed on 2016-05-07 (621 days ago)
InstallationMedia:

JournalErrors:
 Hint: You are currently not seeing messages from other users and the system.
       Users in groups 'adm', 'systemd-journal' can see all messages.
       Pass -q to turn off this notice.
 -- Logs begin at Thu 2018-01-04 17:12:35 UTC, end at Fri 2018-01-19 17:47:53 UTC. --
 -- No entries --
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 XDG_RUNTIME_DIR=<set>
 LANG=en_US.UTF-8
 SHELL=/bin/bash
SourcePackage: python-coverage
UpgradeStatus: Upgraded to bionic on 2017-11-20 (59 days ago)