bpo-36887: add math.isqrt (GH-13244)

bpo-36887: add math.isqrt (GH-13244)

* Add math.isqrt function computing the integer square root.

* Code cleanup: remove redundant comments, rename some variables.

* Tighten up code a bit more; use Py_XDECREF to simplify error handling.

* Update Modules/mathmodule.c

Co-Authored-By: Serhiy Storchaka <storchaka@gmail.com>
* Update Modules/mathmodule.c

Use real argument clinic type instead of an alias

Co-Authored-By: Serhiy Storchaka <storchaka@gmail.com>
* Add proof sketch

* Updates from review.

* Correct and expand documentation.

* Fix bad reference handling on error; make some variables block-local; other tidying.

* Style and consistency fixes.

* Add missing error check; don't try to DECREF a NULL a

* Simplify some error returns.

* Another two test cases:

- clarify that floats are rejected even if they happen to be
  squares of small integers
- TypeError beats ValueError for a negative float

* Documentation and markup improvements; thanks Serhiy for the suggestions!

* Cleaner Misc/NEWS entry wording.

* Clean up (with one fix) to the algorithm explanation and proof.