Update index.py

[seanmacavaney]

Keep IterDictIndexerBase around for backwards compatibility

prevents errors like:

Traceback (most recent call last):                                                                                                                                                    
  File "example.py", line 5, in <module>                                                                                                                                              
    from odis import Prf, ElectraScorer, LexDistil, DirectIndex                                                                                                                       
  File "/home/sean/ws/odis/odis/__init__.py", line 3, in <module>                                                                                                                     
    from .lexdistil import LexDistil                                                                                                                                                  
  File "/home/sean/ws/odis/odis/lexdistil.py", line 8, in <module>                                                                                                                    
    from pyterrier_dr import DocnoFile                                                                                                                                                
  File "/home/sean/miniconda3/envs/py37/lib/python3.7/site-packages/pyterrier_dr/__init__.py", line 1, in <module>                                 
    from .indexes import DocnoFile, NilIndex, NumpyIndex, RankedLists, FaissFlat, FaissHnsw, MemIndex, TorchIndex                                      
  File "/home/sean/miniconda3/envs/py37/lib/python3.7/site-packages/pyterrier_dr/indexes.py", line 114, in <module>                                                 
    class NilIndex(pt.index.IterDictIndexerBase):                                                                                                                                     
AttributeError: module 'pyterrier.index' has no attribute 'IterDictIndexerBase'                                                                                                       

[cmacdonald]

can we add a Deprecated annotation?

[seanmacavaney]

Good call, updated