date-time is not formatted correctly

[a-gertner]

Describe the bug
OpenAPI date-time fields are generated using datetime.datetime.isoformat and don't comply with OpenAPI spec / RFC3339

OpenAPI Spec File

/api/example:
    get:
      tags:
        - "example"
      operationId: example-get
      summary: "summary removed"
      description: ""
      parameters:
      - in: query
        name: "start"
        description: "desciption removed"
        required: true
        schema:
          type: string
          format: date-time
          example: '2020-07-21T17:32:28Z'
      - in: query
        name: "end"
        description: "desciption removed"
        required: true
        schema:
          type: string
          format: date-time
          example: '2020-07-21T17:32:28Z'

Desktop:

• OS: Windows 11
• Python Version: 3.9.13
• openapi-python-client version 0.15.1

Additional context
Client code generated using openapi-python-client version 0.15.1 parses datetime.datetime objects and uses .isoformat() function to convert these to strings. This produces timestamp strings non-compliant with OpenAPI spec (https://github.com/OAI/OpenAPI-Specification/blob/main/versions/3.0.3.md#data-types). According to spec timestamps should look like this:
2023-08-23T13:26:16Z
Instead they look like this:
2023-08-23T12:26:16.979067

I have a server that rejects these as invalid timestamps. I can patch the client manually after generation, but I thought it would be beneficial to submit a bug report.

[dbanty]

If I understand correctly, the only thing isoformat needs in order to be RFC3339-compliant is a timezone—so really what we need is a way to represent datetimes that are timezone-aware, right? I don't know of a way to do that with any builtin types, unfortunately, and building functionality to validate this at runtime won't be a much different experience than having the server respond with an error 🤔.

The "easy" solution is to have users of the generator always pass timezone-aware datetimes 😅. However teaching consumers of the generated clients to do that isn't ideal.

We could create a new DateTime or similar class to use in place of the built-in one, which always requires passing in tzinfo, that feels heavy-handed, though.

Any ideas on a more elegant solution?