bug#26015: [PATCH] python-rpy2: Propagate python2-singledispatch

[Pjotr Prins]

Example of what this patch fixes (sorry for the delay):

Traceback (most recent call last):
  File "runserver.py", line 8, in <module>
    from wqflask import app
  File 
"/gnu/export/local/wrk/izip/git/opensource/genenetwork/sumo_gn2/wqflask/wqflask/__init__.py",
 line 24, in <module>
    import wqflask.views
  File 
"/gnu/export/local/wrk/izip/git/opensource/genenetwork/sumo_gn2/wqflask/wqflask/views.py",
 line 46, in <module>
    from wqflask.marker_regression import marker_regression
  File 
"/gnu/export/local/wrk/izip/git/opensource/genenetwork/sumo_gn2/wqflask/wqflask/marker_regression/marker_regression.py",
 line 16, in <module>
    import rpy2.robjects as ro
  File 
"/home/wrk/opt/genenetwork2-test/lib/python2.7/site-packages/rpy2/robjects/__init__.py",
 line 18, in <module>
    from rpy2.robjects.robject import RObjectMixin, RObject
  File 
"/home/wrk/opt/genenetwork2-test/lib/python2.7/site-packages/rpy2/robjects/robject.py",
 line 8, in <module>
    from . import conversion
  File 
"/home/wrk/opt/genenetwork2-test/lib/python2.7/site-packages/rpy2/robjects/conversion.py",
 line 13, in <module>
    from singledispatch import singledispatch
ImportError: No module named singledispatch