Describe the bug
When generating code with --use-subclass-enum, the resulting code is not type-valid according to mypy

To Reproduce

Example schema:

openapi: 3.0.0
components:
  schemas:
    Unit:
      description: Unit of the data shown
      type: string
      enum:
        - COUNT
        - ORDINAL

generated code:

class Unit(str, Enum):

    count = "COUNT"
    ordinal = "ORDINAL"

Used commandline:

$ datamodel-codegen --use-subclass-enum --input schema.yaml > generated_code.py

mypy generated_code.py failes with the "assignment" error:

$ mypy generated_code.py 
generated_code.py:14: error: Incompatible types in assignment (expression has type "str", base class "str" defined the type as "Callable[[str, str, Optional[SupportsIndex], Optional[SupportsIndex]], int]")  [assignment]

Expected behavior
mypy generated_code.py should pass

Version:

• OS: Mac
• Python version: 3.9
• datamodel-code-generator version: 0.18.0