readthedocs.org: autodoc fails because of missing namespace package

Details

• Project URL: https://readthedocs.org/projects/metapensierosignal
• Build URL (if applicable): https://readthedocs.org/projects/metapensierosignal/builds/6276037/
• Read the Docs username (if applicable): azazel75

Expected Result

That autodoc fills pages like http://metapensierosignal.readthedocs.io/en/latest/core.html with some content

Actual Result

Due to this import errors (that I failed to reproduce locally) showed in the log of the fifth step of the build at the url above:

Running Sphinx v1.5.6
making output directory...
loading translations [en]... done
loading pickled environment... not yet created
building [mo]: targets for 0 po files that are out of date
building [readthedocs]: targets for 8 source files that are out of date
updating environment: 8 added, 0 changed, 0 removed
reading sources... [ 12%] api
reading sources... [ 25%] core
reading sources... [ 37%] external
reading sources... [ 50%] index
reading sources... [ 62%] intro
reading sources... [ 75%] user
reading sources... [ 87%] utils
reading sources... [100%] weak

/home/docs/checkouts/readthedocs.org/user_builds/metapensierosignal/checkouts/latest/doc/core.rst:19: WARNING: autodoc: failed to import class 'Signal' from module 'metapensiero.signal.core'; the following exception was raised:
Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/metapensierosignal/envs/latest/local/lib/python3.5/site-packages/sphinx/ext/autodoc.py", line 560, in import_object
    __import__(self.modname)
ImportError: No module named 'metapensiero'
...

the api docs files like http://metapensierosignal.readthedocs.io/en/latest/core.html show that no doc is being outputted by autodoc

Anyone as any clue how to even reproduce this?