Bug#902478: [skimage] broken documentation package

[Lumin]

Package: python-skimage-doc
Version: 0.14.0-1
Severity: normal

from 0.14.0-1 debian/rules:

```
@@ -15,0 +16,2 @@ skimage (0.14.0-1) unstable; urgency=medium
     cd doc; test -d build/html || $(MAKE) html
     cd doc; test -d build/html || $(MAKE) html PYTHON=python3
```

This actually didn't work properly, because sphinx (python3)
failed to grab python3 package from python2 import path.

```
export PYTHONPATH=$(PKG_TMP)/usr/lib/python$(PYVER)/dist-packages;
```

Consequence: API documents are missing from python-skimage-doc.

reference solution:

   export PYTHONPATH=$(PKG_TMP)/usr/lib/python3/dist-packages;

reference buildlog:

   : # Build Documentation
   export PYTHONPATH=/<<PKGBUILDDIR>>/debian/tmp/usr/lib/python2.7/dist-packages; \
    cd doc; test -d build/html || /usr/bin/make html PYTHON=python3
   make[2]: Entering directory '/<<PKGBUILDDIR>>/doc'
   python3 tools/build_modref_templates.py
   *WARNING* API documentation not generated: Can not import skimage
   Build API docs...done.
   Copying release notes
   python /usr/bin/sphinx-build -b html -d build/doctrees  -j 1 source build/html
   Running Sphinx v1.7.5
   making output directory...
   loading pickled environment... not yet created

   http://debomatic-amd64.debian.net/distribution#unstable/skimage/0.14.0-1/buildlog