mark poisson_low() function for removal

[GavinHuttley]

What should be changed

cogent3.maths.stats.poisson_low()

Why

Not used in cogent3, available in scipy.stats?

How much notice

6 months notice. This should be represented in the user warning as a specific version. e.g. .<current month + x> where x is from the PR date.

Guidelines for deprecation

As this is a function that will be removed, see dev guidelines for deprecation of code. Indicate the limiting version (for when the old function will be removed) as the number above from when you make the change.

[Zongjing-Han]

I would like to do this issue