Requirements are not automatically installed when running a pip install

[arokem]

When installing into a Docker image, we get:

Python 3.8.10 (default, May 12 2021, 15:46:43) 
[GCC 8.3.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import picard
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python3.8/site-packages/picard/__init__.py", line 23, in <module>
    from .dropin_sklearn import Picard  # noqa
  File "/usr/local/lib/python3.8/site-packages/picard/dropin_sklearn.py", line 10, in <module>
    from sklearn.decomposition import FastICA
ModuleNotFoundError: No module named 'sklearn'

I think that's because the setup.py doesn't have a "requires" section, but not sure.

[pierreablin]

Thanks ! See #42 :)