Fixed tests when using Zope 5.11.

In one test we got a `mappingproxy` instead of a dictionary:

```
File "...plone.base-2.0.1-py3.10.egg/plone/base/tests/messages.rst", line 39, in messages.rst
Failed example:
    msg.mapping
Expected:
    {'name': 'Plone'}
Got:
    mappingproxy({'name': 'Plone'})
```

news/956.tests

@@ -0,0 +1,3 @@
+Fixed tests when using Zope 5.11.
+In one test we got a `mappingproxy` instead of a dictionary.
+[maurits]

src/plone/base/tests/messages.rst

@@ -36,7 +36,7 @@ And at last there is the possibility of variable substitution:
   >>> msg.default
   'Hello ${name}'
 
-  >>> msg.mapping
+  >>> dict(msg.mapping)
   {'name': 'Plone'}
 
 Messages with translation service set up