Add check-package step also to 'test' workflow

This ensures we do not have any surprises when we try to deploy,
which uses the same step to generate the package.

setup.cfg

@@ -2,6 +2,7 @@
 name = pytest-xdist
 description = pytest xdist plugin for distributed testing, most importantly across multiple CPUs
 long_description = file: README.rst
+long_description_content_type = text/x-rst
 license = MIT
 author = holger krekel and contributors
 author_email = pytest-dev@python.org,holger@merlinux.eu