chore: drop support for PyPy < 7.3.1 and clean up old PyPy workarounds (#2456)

* Remove code inside 'PYPY_VERSION_NUM < 0x06000000' preprocessor if branch

* fix: more cleanup

* Remove more references to PyPy 5.7 and 5.9 in the docs

* Update comment on PyUnicode_UTF* in PyPy

Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>