date-time is not formatted correctly

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.

首先定位处理 datetime.datetime 值并调用 isoformat() 的生成客户端请求序列化代码。将其输出与 issue 中的 RFC3339/OpenAPI 示例进行比较，然后运行或补充对 date-time 查询参数的覆盖。完成的标准是，生成的时间戳被报告中的服务器接受为符合规范的 date-time 值。