intervene: fix pandas version <1.0

[bernt-matthias]

in pandas 1.0 DataFrame.ix was deprecated (pandas-dev/pandas#26438) which leads to:

  File "/usr/local/lib/python3.8/site-packages/intervene/modules/pairwise/pairwise.py", line 214, in heatmap_triangle
    D = D.ix[cluster_order, cluster_order]
  File "/usr/local/lib/python3.8/site-packages/pandas/core/generic.py", line 5139, in __getattr__
    return object.__getattribute__(self, name)

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[bernt-matthias]

@BiocondaBot please add label

[dpryan79]

@bioconda-bot please merge

[BiocondaBot]

I will attempt to upload artifacts and merge this PR. This may take some time, please have patience.