bpo-45876: Improve accuracy for stdev() and pstdev() in statistics (GH-29736)

* Inlined code from variance functions

* Added helper functions for the float square root of a fraction

* Call helper functions

* Add blurb

* Fix over-specified test

* Add a test for the _sqrt_frac() helper function

* Increase the tested range

* Add type hints to the internal function.

* Fix test for correct rounding

* Simplify ⌊√(n/m)⌋ calculation

Co-authored-by: Mark Dickinson <dickinsm@gmail.com>

* Add comment and beef-up tests

* Test for zero denominator

* Add algorithmic references

* Add test for the _isqrt_frac_rto() helper function.

* Compute the 109 instead of hard-wiring it

* Stronger test for _isqrt_frac_rto()

* Bigger range

* Bigger range

* Replace float() call with int/int division to be parallel with the other code path.

* Factor out division. Update proof link. Remove internal type declaration

Co-authored-by: Mark Dickinson <dickinsm@gmail.com>