fix?: recursion error on Sanic subclass initialisation (#2072)

* fix?: recursion error on Sanic subclass init

* tests: add test case for sanic subclass initialisation

* Remove BaseSanic metaclass

Co-authored-by: Adam Hopkins <admhpkns@gmail.com>

tests/test_app.py

@@ -405,3 +405,10 @@ def test_app_set_context(app):
 
     retrieved = Sanic.get_app(app.name)
     assert retrieved.ctx.foo == 1
+
+
+def test_subclass_initialisation():
+    class CustomSanic(Sanic):
+        pass
+
+    CustomSanic("test_subclass_initialisation")