Fix type for GroupConcat arg 1

Author: adamchainz

src/django_mysql/models/aggregates.py

@@ -5,7 +5,6 @@
 from django.db.backends.base.base import BaseDatabaseWrapper
 from django.db.models import Aggregate
 from django.db.models import CharField
-from django.db.models import Expression
 from django.db.models.sql.compiler import SQLCompiler
 
 
@@ -29,7 +28,7 @@ class GroupConcat(Aggregate):
 
     def __init__(
         self,
-        expression: Expression,
+        expression: Any,
         distinct: bool = False,
         separator: str | None = None,
         ordering: str | None = None,