use hatch instead of setuptools

hatch is the new recommended build system for python packages promoted by the PyPA.

• from user perspective nothing changes (you can use the hatch command now)
• no MANIFEST.in needed anymore (will ship .gitignore in sdist to be able to build wheel, auto-included)
• license-files entry in [project] not needed (will automatically determine common license file names)
• added .git_archival.txt and .gitattributes for git archive install support (correct version determined), solves #99 (closed)

Question

• Do we want more explicit dev versions? (0.4.0rc3.dev566+ga16a6c6 vs 0.4.0rc3.dev566)