Fix mypy plugin issue with self field (#2743)

* Fix mypy issue with self field

* Add docs

changes/2743-uriyyo.md

@@ -0,0 +1 @@
+Fix mypy plugin issue with self field declaration.

pydantic/mypy.py

@@ -619,7 +619,7 @@ def add_method(
     #     first = []
     else:
         self_type = self_type or fill_typevars(info)
-        first = [Argument(Var('self'), self_type, None, ARG_POS)]
+        first = [Argument(Var('__pydantic_self__'), self_type, None, ARG_POS)]
     args = first + args
     arg_types, arg_names, arg_kinds = [], [], []
     for arg in args:

tests/mypy/modules/plugin_success.py

@@ -158,3 +158,7 @@ class NotFrozenModel(FrozenModel):
 
 NotFrozenModel(x=1).x = 2
 NotFrozenModel.from_orm(model)
+
+
+class ModelWithSelfField(BaseModel):
+    self: str