Prepare libhxl-python 5.0 release

[davidmegginson]

• [ ] ensure no open pull requests
• [ ] run unit tests in dev
• [ ] update version to "5.0" in setup.py and hxl/__init__.py
• [ ] check CHANGELOG
• [ ] install and test in clean virtualenv
• [ ] commit changes
• [ ] merge to test and push
• [ ] merge to main and push
• [ ] create libhxl PyPi release: pip install twine && python setup.py sdist && twine upload dist/libhxl-5.0*
• [ ] wait until the new release is visible on PyPi https://pypi.org/project/libhxl/5.0/
• [ ] add git tag "5.0"
• [ ] create GitHub release

[davidmegginson]

Moved into a Google Doc: https://docs.google.com/document/d/1She9AAPGLy410N7f1eCmojOvXuHBIH2WAS1ljJXv1KA/edit?usp=sharing