Stange import error for nibabel when trying to import from .pybuild

[Andreas Tille <andreas@an3as.eu>]

Hi,

to see what I can do about bug #909990 I've imported the latest version
into the packaging Git[1].  When beeing inside the main dir of the Git
repository I can easily do

neurodebian-team/pynifti(debian) $ python3
Python 3.6.6 (default, Jun 27 2018, 14:44:17) 
[GCC 8.1.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import nibabel
>>>

However, if I try inside the .pybuild dir I get:

/build/nibabel-2.3.0/.pybuild/cpython3_3.6_nibabel/build# python3.6
Python 3.6.7rc1 (default, Sep 27 2018, 09:51:25) 
[GCC 8.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import nibabel
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/build/nibabel-2.3.0/.pybuild/cpython3_3.6_nibabel/build/nibabel/__init__.py", line 45, in <module>
    from .loadsave import load, save
  File "/build/nibabel-2.3.0/.pybuild/cpython3_3.6_nibabel/build/nibabel/loadsave.py", line 18, in <module>
    from .imageclasses import all_image_classes
  File "/build/nibabel-2.3.0/.pybuild/cpython3_3.6_nibabel/build/nibabel/imageclasses.py", line 16, in <module>
    from .minc1 import Minc1Image
  File "/build/nibabel-2.3.0/.pybuild/cpython3_3.6_nibabel/build/nibabel/minc1.py", line 20, in <module>
    from .deprecated import FutureWarningMixin
ImportError: cannot import name 'FutureWarningMixin'
>>>      


This leads to errors when the build process is trying to create the
docs via

  export PYTHONPATH=`pybuild --print build_dir --interpreter python3` ; \
  cd doc && /usr/bin/make html-only

(which triggers exactly the above error message).

Any idea what might went wrong here?  Its definitely not the slightly
different Python 3.6 version - I tried a local build using 3.6.6 with
the very same effect.

Kind regards

       Andreas.

[1] Vcs-Git: https://salsa.debian.org/neurodebian-team/pynifti.git -b debian
    (yes, I switched to Branch debian instead of current package 2.3.0-1
     which has non-default Branch dist/debian/proper)

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