Prepare release of v0.18.0 (#282)

Signed-off-by: Michael Seifert <m.seifert@digitalernachschub.de>
Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>

Author: seifertm

README.rst

@@ -257,7 +257,7 @@ or an async framework such as `asynctest <https://asynctest.readthedocs.io/en/la
 Changelog
 ---------
 
-0.18.0 (Unreleased)
+0.18.0 (22-02-07)
 ~~~~~~~~~~~~~~~~~~~
 
 - Raise a warning if @pytest.mark.asyncio is applied to non-async function. `#275 <https://github.com/pytest-dev/pytest-asyncio/issues/275>`_