lib.is_scalar misses PEP 3141 numbers
Looking at `np.isscalar`'s docstring I see:

```
    NumPy supports PEP 3141 numbers:

    >>> from fractions import Fraction
    >>> isscalar(Fraction(5, 17))
    True
    >>> from numbers import Number
    >>> isscalar(Number())
    True
```

pandas `lib.is_scalar` doesn't recognize Number or Fraction.