Both references were referencing links from Python because of our intersphinx
mapping in `conf.py`:
intersphinx_mapping = {"python": ("https://docs.python.org/3", None)}
Because Python's docs explicitly define both references, Sphinx fallbacks to
them instead of generating implicit references as was expected.
Fix#6397
Now `tox -e docs` will also include the draft changelog for the
next version (locally only).
`CHANGELOG.rst` now only points to the changelog on READTHEDOCS so
sphinx diretives can be used.
Followup to https://github.com/pytest-dev/pytest/pull/6272