Prepare release of v0.20.0.

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

Author: seifertm

CHANGELOG.rst

@@ -2,7 +2,7 @@
 Changelog
 =========
 
-UNRELEASED
+0.20.0 (22-10-21)
 =================
 - BREAKING: Removed *legacy* mode. If you're upgrading from v0.19 and you haven't configured ``asyncio_mode = legacy``, you can upgrade without taking any additional action. If you're upgrading from an earlier version or you have explicitly enabled *legacy* mode, you need to switch to *auto* or *strict* mode before upgrading to this version.
 - Deprecate use of pytest v6.