Restrict back to < 20.8b0 for now

Signed-off-by: Pierre-Yves Chibon <pingou@pingoured.fr>

requirements-testing.txt

@@ -1,6 +1,6 @@
 bcrypt
 beautifulsoup4
-black; python_version >= '3.6' # Only available on py3.6+
+black == 19.10b0; python_version >= '3.6' # Only available on py3.6+
 cryptography
 eventlet
 fedmsg