Initial path to handle new pandas error

This prevents attempting to drop columns that don't exist in merged.columns
after setting the index, while still dropping columns that are present in
merged.columns.  Attempting to do see raises an exception in pandas >= 1.

Please see pandas-dev/pandas#8594
for details.

Author: andrewsanchez

q2_diversity/_alpha/_visualizer.py

@@ -241,7 +241,10 @@ def _reindex_with_metadata(column, columns, merged):
     merged.sort_index(axis=0, ascending=True, inplace=True)
     merged = merged.groupby(level=[column])
     counts = merged.count()
-    counts.drop(columns, axis=1, inplace=True, level=0)
+    # Removes the column name used to set the index of `merged` above
+    col_diff = set(columns) - set([column])
+    if col_diff:
+        counts.drop(col_diff, axis=1, inplace=True, level=0)
     median_ = merged.median()
     return median_, counts