[3.12] gh-71339: Add additional assertion methods in test.support (GH-128707) (GH-128815) (GH-129059)

Add a mix-in class ExtraAssertions containing the following methods:

* assertHasAttr() and assertNotHasAttr()
* assertIsSubclass() and assertNotIsSubclass()
* assertStartsWith() and assertNotStartsWith()
* assertEndsWith() and assertNotEndsWith()

(cherry picked from commit c6a566e47b9903d48e6e1e78a1af20e6c6c535cf)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
(cherry picked from commit 06cad77a5b345adde88609be9c3c470c5cd9f417)