Pyrsistent

* New type PClass, a persistent version of a Python object. Related to issues 30 and 32.
Thanks exarkun and radix for input on this one!
* Rename PRecordTypeError -> PTypeError, it is now also raised by PClass
* New convenience functions, pvector_field, pmap_field and pset_field to create PRecord/PClass
fields for checked collections. Issues 26 and 36. Thanks to itamarst for this!
* Removed deprecated function set_in() on PMap and PVector.
* Removed deprecated factory function pclass.
* Major internal restructuring breaking pyrsistent.py into multiple files. This should
not affect those only using the public interface but if you experience problems please
let me know.

* Fix 245, never introduce new nodes during discard.
* Fix 268, do not rely on well implemented __ne__ for keys in pmaps, instead do explicit inversion of equality
comparison when checking for inequality.
* Officially support Python 3.12.
* Officially drop support for Python 3.7.
* Fix 273, build more types of wheels. Thanks jams2 for this!
* Fix 282, add generic types to types. Thanks lukasK9999 for this!
* Fix 281, defaultdict can now be frozen. NB! This is a backwards incompatible fix since defaultdict was not
previously frozen.

* Fix 264, add wheels and official support for Python 3.11. Thanks hugovk for this!

* Fix 263, pmap regression in 0.19.1. Element access sometimes unreliable after insert.
Thanks mwchase for reporting this!

* Fix 159 (through PR 243). Pmap keys/values/items now behave more like the corresponding Python 3
methods on dicts. Previously they returned a materialized PVector holding the items, now they return
views instead. This is a slight backwards incompatibility compared to previous behaviour, hence stepping
version to 0.19. Thanks noahbenson for this!
* Fix 244, type for argument to PVector.delete missing. thanks dscrofts for this!
* Fix 249, rename perf test directory to avoid tripping up automatic discovery in more recent setuptools versions
* Fix 247, performance bug when setting elements in maps and adding elements to sets
* Fix 248, build pure Python wheels. This is used by some installers. Thanks andyreagan for this!
* Fix 254, 258, support manylinux_2014_aarch64 wheels. Thanks Aaron-Durant for this!