Since python setup.py sdist bdist_wheel is deprecated, this uses build instead. Also, it turns out, we forgot to add our requirements.txt to the manifest (and ocrd.processor.builtin.dummy as a package) for correct packaging. Further improves the makefile.

Note: in the long run we should

• switch to pyproject.toml, with setuptools as build backend, and setup.cfg
• move requirements.txt to the latter's install_requires key, and either abandon requirements.txt or keep it for dev purposes (with a workable state of pip freeze)
• tie versioning to git tags (so there will be no need for manual synchronization anymore, and not interdependencies at build time) via setuptools_scm recipe – see this example