Docs don't build on Python3

[lafrech]

I just tried building the docs with only python3-sphinx installed and it failed.

$ make html
sphinx-build -b html -d _build/doctrees   . _build/html
Making output directory...
Running Sphinx v1.2.3

Exception occurred:
  File "/home/jerome/projets/mongodb/mongoengine/mongoengine/mongoengine/__init__.py", line 1, in <module>
    import document
ImportError: No module named 'document'

With python-sphinx installed, it succeeded.

I guess this is because 2to3 is not used when building the docs.

Not sure how to address this, especially since @thedrow says:

I think we should migrate from 2to3 eventually or at least greatly reduce it's usage.

[wojcikstefan]

This was fixed by #1428, which moved all files to absolute imports.