refactor!: remove BaseQuerySet from base module

BaseQuerySet is now defined only in the synchronous queryset implementation.

Author: arunsureshkumar

mongoengine/asynchronous/queryset/base.py

@@ -1147,8 +1147,8 @@ def select_related(self, *fields: str):
             *fields: dotted paths of reference fields to preload.
                      Examples:
                         select_related("author")
-                        select_related("author.country")
-                        select_related("comments.user")
+                        select_related("author__country")
+                        select_related("comments__user")
 
         Returns:
             QuerySet — clone with select_related instructions