Bump isort, use --diff (#164)

* dev-requirements: bump isort: 4.3.4 => 4.3.21

* isort: use --diff

* known_first_party = mypy_django_plugin

* Run isort

mypy_django_plugin/lib/helpers.py

@@ -1,15 +1,19 @@
 from collections import OrderedDict
-from typing import Any, Dict, List, Optional, Set, TYPE_CHECKING, Union, cast
+from typing import TYPE_CHECKING, Any, Dict, List, Optional, Set, Union, cast
 
 from mypy import checker
 from mypy.checker import TypeChecker
 from mypy.mro import calculate_mro
-from mypy.nodes import (Block, ClassDef, Expression, GDEF, MDEF, MemberExpr, MypyFile, NameExpr, StrExpr, SymbolNode,
-                        SymbolTable, SymbolTableNode, TypeInfo, Var)
+from mypy.nodes import (
+    GDEF, MDEF, Block, ClassDef, Expression, MemberExpr, MypyFile, NameExpr, StrExpr, SymbolNode, SymbolTable,
+    SymbolTableNode, TypeInfo, Var,
+)
 from mypy.plugin import (
     AttributeContext, CheckerPluginInterface, FunctionContext, MethodContext,
 )
-from mypy.types import AnyType, Instance, NoneTyp, TupleType, Type as MypyType, TypeOfAny, TypedDictType, UnionType
+from mypy.types import AnyType, Instance, NoneTyp, TupleType
+from mypy.types import Type as MypyType
+from mypy.types import TypedDictType, TypeOfAny, UnionType
 
 from mypy_django_plugin.lib import fullnames