BUG: Provide a class name to the Sphinx lexer

Provide a class name to the `Sphinx` lexer: the `add_lexer` method does
not accept lexer instances as an argument since version 4.0.

Fixes:
```
Exception occurred:
  File ".virtualenvs/tract_querier/lib/python3.10/site-packages/sphinx/highlighting.py", line 159, in get_lexer
    lexer = lexer_classes[lang](**opts)
TypeError: 'WMQLLexer' object is not callable
```

raised in a local build with Python 3.10 and Sphinx 8.1.3.

Documentation:
https://www.sphinx-doc.org/en/master/extdev/appapi.html#sphinx.application.Sphinx.add_lexer

doc/conf.py

@@ -259,5 +259,4 @@ def get_tokens_unprocessed(self, text):
                     yield index, Keyword.Pseudo, value
                 else:
                     yield index, token, value
-    wmql_lexer = WMQLLexer()
-    app.add_lexer('wmql', wmql_lexer)
+    app.add_lexer('wmql', WMQLLexer)