Fix towncrier output

[The-Compiler]

Add top line to towncrier template

Looks correct now:

.. towncrier release notes start

pytest 7.0.0 (2021-08-27)
=========================


Breaking Changes
----------------

- `#8246 <https://github.com/pytest-dev/pytest/issues/8246>`_: ``--version`` now writes version information to ``stdout`` rather than ``stderr``.
[...]

Fixes #8817, supersedes #9045 and also includes the changelog commit cherry-picked from there.

See twisted/towncrier#346 and twisted/towncrier#340.

[nicoddemus]

Awesome! 😁

[The-Compiler]

Argh, now the docs build fails with:

/home/runner/work/pytest/pytest/doc/en/_changelog_towncrier_draft.rst:2: WARNING: Document or section may not begin with a transition.

probably because {top_line} isn't in the template. But when I add {top_line}, running towncrier manually ends up in having the version line there twice. I really don't understand how towncrier works...

[nicoddemus]

I added the changelog entry in #9047 to get the ball rolling for 6.2.5, so you can remove that commit from here. 👍

[The-Compiler]

Just took another look, and it looks like we will need to wait for towncrier 21.9.0: twisted/towncrier#360

Will open another PR to pin towncrier to an older version, similar to what @nicoddemus proposed.