provide long_description_content_type in setup.py of all projects

[gdementen]

During release we get this warning:

twine check dist/*
Checking dist\larray_eurostat-0.34-py3-none-any.whl: PASSED, with warnings
warning: long_description_content_type missing. defaulting to text/x-rst.
Checking dist\larray_eurostat-0.34.tar.gz: PASSED, with warnings
warning: long_description_content_type missing. defaulting to text/x-rst.