Describe the bug

It produces a Enumeration with key with None_ prepended and without quotes around the value

Traceback (most recent call last):
  File "/Users/tomhemmes/miniconda3/envs/py38/lib/python3.8/site-packages/datamodel_code_generator/__main__.py", line 244, in main
    generate(
  File "/Users/tomhemmes/miniconda3/envs/py38/lib/python3.8/site-packages/datamodel_code_generator/__init__.py", line 202, in generate
    results = parser.parse()
  File "/Users/tomhemmes/miniconda3/envs/py38/lib/python3.8/site-packages/datamodel_code_generator/parser/base.py", line 354, in parse
    body = format_code(body, self.target_python_version)
  File "/Users/tomhemmes/miniconda3/envs/py38/lib/python3.8/site-packages/datamodel_code_generator/format.py", line 26, in format_code
    code = apply_black(code, python_version)
  File "/Users/tomhemmes/miniconda3/envs/py38/lib/python3.8/site-packages/datamodel_code_generator/format.py", line 41, in apply_black
    return black.format_str(
  File "/Users/tomhemmes/miniconda3/envs/py38/lib/python3.8/site-packages/black.py", line 725, in format_str
    src_node = lib2to3_parse(src_contents.lstrip(), mode.target_versions)
  File "/Users/tomhemmes/miniconda3/envs/py38/lib/python3.8/site-packages/black.py", line 836, in lib2to3_parse
    raise exc from None
black.InvalidInput: Cannot parse: 10:23:     None_Item_1 = Item 1

To Reproduce

Example schema:

{
  "$id": "item_types.json",
  "$schema": "http://json-schema.org/draft-07/schema#",
  "title": "item_types",
  "description": "JSON Schema for item_types",
  "type": "array",
  "items": {
    "enum": [
      "Item 1",
      "Item 2",
      "Item 3"
    ]
  }
}

Used commandline:

$ datamodel-codegen --input test.json --input-file-type jsonschema --output test.py

Expected behavior
A clear and concise description of what you expected to happen

Version:

• OS: macOS Catalina 10.15.4 (19E287)
• Python version: Python 3.8
• datamodel-code-generator version: datamodel-codegen --version
  0.6.2

Additional context
Add any other context about the problem here.