Update package to 2.1.0. This is a major version bump with API changes
including new callback signatures.
Add PYPI_SOURCE_NAME since PyPI switched sdist filename to use
underscores (paho_mqtt-2.1.0.tar.gz).
Add PKG_BUILD_DEPENDS for python-hatchling since upstream switched
build backend from setuptools to hatchling.
Add test.sh to verify basic client functionality.
Signed-off-by: Alexandru Ardelean <alex@shruggie.ro>