bpo-30794: added kill() method to multiprocessing.Process (#2528)

* bpo-30794: added kill() method to multiprocessing.Process

* Added entries to documentation and NEWS

* Refactored test_terminate and test_kill

* Fix SIGTERM and SIGKILL being used on Windows for the tests

* Added "versionadded" marker to the documentation

* Fix trailing whitespace in doc
6 files changed