CI: Reject Ansible 12.0a6, avoids bug in yaml callback plugin

Avoids an error when the yaml stdout callback is enabled

> module 'ansible._internal._yaml._dumper' has no attribute 'SafeRepresenter'

refs #1284

tox.ini

@@ -89,6 +89,9 @@ deps =
     ansible10: ansible~=10.0
     ansible11: ansible~=11.0
     ansible12: ansible>=12.0a
+    # Avoid yaml callback bug, https://github.com/mitogen-hq/mitogen/issues/1284
+    ansible12: ansible!=12.0a6
+    ansible12: ansible-core!=2.19b6
 install_command =
     python -m pip --no-python-version-warning --disable-pip-version-check install {opts} {packages}
 commands_pre =