Make Index.all compatible with numpy.all
- [ ] closes #xxxx
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

Due to a bug in my own code, I discovered that `pandas.Index` objects cannot be passed to the `numpy.all` function:

```python
In [1]: import numpy as np

In [2]: import pandas as pd

In [3]: idx = pd.Int64Index([1,2,3,4,5])

In [4]: np.all(idx)
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-4-597ac8c43e48> in <module>
----> 1 np.all(idx)

<__array_function__ internals> in all(*args, **kwargs)

~/venvs/funpack.venv/lib/python3.7/site-packages/numpy/core/fromnumeric.py in all(a, axis, out, keepdims, where)
   2437     """
   2438     return _wrapreduction(a, np.logical_and, 'all', axis, None, out,
-> 2439                           keepdims=keepdims, where=where)
   2440
   2441

~/venvs/funpack.venv/lib/python3.7/site-packages/numpy/core/fromnumeric.py in _wrapreduction(obj, ufunc, method, axis, dtype, out, **kwargs)
     83                 return reduction(axis=axis, dtype=dtype, out=out, **passkwargs)
     84             else:
---> 85                 return reduction(axis=axis, out=out, **passkwargs)
     86
     87     return ufunc.reduce(obj, axis, dtype, out, **passkwargs)

TypeError: all() got an unexpected keyword argument 'axis'
```

Using [this PR](https://github.com/pandas-dev/pandas/pull/26324/) as precedent, the fix is quite straightforward. 
