bpo-29962: add math.remainder (#950)

bpo-29962: add math.remainder (#950)

* Implement math.remainder.

* Fix markup for arguments; use double spaces after period.

* Mark up function reference in what's new entry.

* Add comment explaining the calculation in the final branch.

* Fix out-of-order entry in whatsnew.

* Add comment explaining why it's good enough to compare m with c, in spite of possible rounding error.