Use lowercase hex numbers fixes #1692 (#1775)

* Made hex lower case

* Refactored numeric formatting section

* Redid some refactoring and removed bloat

* Removed additions from test_requirements.txt

* Primer now expects expected changes

* Undid some refactoring

* added to changelog

* Update src/black/__init__.py

Co-authored-by: Zsolt Dollenstein <zsol.zsol@gmail.com>

Co-authored-by: Zsolt Dollenstein <zsol.zsol@gmail.com>
Co-authored-by: Cooper Lees <me@cooperlees.com>