docs: Prepare v0.19.0 release. (#385)

Signed-off-by: Michael Seifert <m.seifert@digitalernachschub.de>

Author: seifertm

CHANGELOG.rst

@@ -2,7 +2,7 @@
 Changelog
 =========
 
-UNRELEASED
+0.19.0 (22-07-13)
 =================
 - BREAKING: The default ``asyncio_mode`` is now *strict*. `#293 <https://github.com/pytest-dev/pytest-asyncio/issues/293>`_
 - Removes `setup.py` since all relevant configuration is present `setup.cfg`. Users requiring an editable installation of pytest-asyncio need to use pip v21.1 or newer. `#283 <https://github.com/pytest-dev/pytest-asyncio/issues/283>`_