[SciPy-Dev] special.factorial accepting floats

[Andrew Nelson]

I came across this because I was setting up a CI run against Python3.9. On
Py3.9 there is a DeprecationWarning if `math.factorial` is supplied a float:

On Py3.8:
math.factorial(2.0) --> 2

On Py3.9:
math.factorial(2.0) --> 2 and emits a "DeprecationWarning: Using
factorial() with floats is deprecated"

Presumably with Py >> 3.9:
math.factorial(2.0) will raise an Exception

As you say, `math.factorial` is used by `special.factorial` for
`exact=True`. When Python3.9 is released various tests will start to fail
because of that DeprecationWarning.
I know it's early days, I'm just trying to be proactive.

- how to modify tests to account for that DeprecationWarning when
`special.factorial` is supplied a float with `exact=True`
- how test code and special.factorial code has to be modified for a post
DeprecationWarning period.
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