Categorical.fillna doesn't accept tuples

[TomAugspurger]

This should maybe work.

In [1]: import pandas as pd

In [2]: pd.Categorical([(1, 2), None]).fillna((1, 2))
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-2-73c5b4ab00c2> in <module>()
----> 1 pd.Categorical([(1, 2), None]).fillna((1, 2))

~/sandbox/pandas-ip/pandas/pandas/util/_decorators.py in wrapper(*args, **kwargs)
    136                 else:
    137                     kwargs[new_arg_name] = new_arg_value
--> 138             return func(*args, **kwargs)
    139         return wrapper
    140     return _deprecate_kwarg

~/sandbox/pandas-ip/pandas/pandas/core/arrays/categorical.py in fillna(self, value, method, limit)
   1664                 raise TypeError('"value" parameter must be a scalar, dict '
   1665                                 'or Series, but you passed a '
-> 1666                                 '"{0}"'.format(type(value).__name__))
   1667
   1668         return self._constructor(values, categories=self.categories,

TypeError: "value" parameter must be a scalar, dict or Series, but you passed a "tuple"

xref #19684

This could be a can of worms if we want to accept Series(categorical).fillna(tuple), since it's not allowed there. Would accept incremental improvements here of course.