Move custom_admonitions monkey patching out of extension setup function

This ensures it is not redundantly patched if the extension is loaded
more than once in a given process (only really relevant for tests).

sphinx_immaterial/custom_admonitions.py

@@ -148,6 +148,10 @@ def depart_admonition(self: HTML5Translator, node: Optional[nodes.Element] = Non
     HTML5Translator.depart_admonition = depart_admonition  # type: ignore[assignment]
 
 
+patch_visit_admonition()
+patch_depart_admonition()
+
+
 class CustomAdmonitionDirective(Directive, ABC):
     """A base class to define custom admonition directives.
 
@@ -364,9 +368,6 @@ def setup(app: Sphinx):
     app.connect("env-check-consistency", add_admonition_and_icon_css)
     app.connect("config-inited", on_config_inited)
 
-    patch_visit_admonition()
-    patch_depart_admonition()
-
     return {
         "parallel_read_safe": True,
         "parallel_write_safe": True,