[BUG][Python] Client generation doesn't handle regex pattern correctly

Created by: aaronatbissell

Bug Report Checklist

Have you provided a full/minimal spec to reproduce the issue?
Have you validated the input using an OpenAPI validator (example)?
Have you tested with the latest master to confirm the issue still exists?
Have you searched for related issues/PRs?
What's the actual output vs expected output?
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Description

When generating a python client library where a string pattern like this is present, the validation logic in the client code is incorrect

datetime:
  type: string
  pattern: '[\d]{4}-[\d]{2}-[\d]{2}T[\d]{1,2}:[\d]{2}Z'

openapi-generator version

5.4.0

OpenAPI declaration file content or url

openapi: 3.0.0
info:
  description: a spec to test free form object models
  version: 1.0.0
  title: OpenAPI Petstore
  license:
    name: Apache-2.0
    url: "https://www.apache.org/licenses/LICENSE-2.0.html"
tags: []
paths: {}
components:
  schemas:
    DateTimeObject:
      properties:
        datetime:
          type: string
          pattern: '[\d]{4}-[\d]{2}-[\d]{2}T[\d]{1,2}:[\d]{2}Z'

Generation Details

openapi-generator generate -i spec.yaml -g python

This generates a validation string of:

validations = {
  ('datetime',): {
    'regex': {
      'pattern': r'[d]{4}-[\d]{2}-[\d]{2}T[\d]{1,2}:[\d]{2}Z/', # noqa: E501
    },
  },
}

Steps to reproduce

Using a config like this:

openapi: 3.0.0
info:
  description: a spec to test free form object models
  version: 1.0.0
  title: OpenAPI Petstore
  license:
    name: Apache-2.0
    url: "https://www.apache.org/licenses/LICENSE-2.0.html"
tags: []
paths: {}
components:
  schemas:
    DateTimeObject:
      properties:
        datetime:
          type: string
          pattern: '[\d]{4}-[\d]{2}-[\d]{2}T[\d]{1,2}:[\d]{2}Z'

Then running

openapi-generator generate -i spec.yaml -g python

This generates a validation string of:

validations = {
  ('datetime',): {
    'regex': {
      'pattern': r'[d]{4}-[\d]{2}-[\d]{2}T[\d]{1,2}:[\d]{2}Z/', # noqa: E501
    },
  },
}

and it should be

r'[\d]{4}-[\d]{2}-[\d]{2}T[\d]{1,2}:[\d]{2}Z'

Related issues/PRs

#1823 (closed) - related but for the python-server code base #8098 - similar, but python-flask and specific to escaped characters

Suggest a fix

I'll try to post a PR soon