使用 requests.post 可以发送一个请求，参数是纯文本字符串。
requests.post 参数
def post(url, data=None, json=None, **kwargs):
r"""Sends a POST request.
:param url: URL for the new :class:`Request` object.
:param data: (optional) Dictionary, list of tuples, bytes, or file-like
object to send in the body of the :class:`Request`.
:param json: (optional) json data to send in the body of the :class:`Request`.
:param \*\*kwargs: Optional arguments that ``request`` takes.
:return: :class:`Response <Response>` object
:rtype: requests.Response
"""
return request('post', url, data=data, json=json, **kwargs)
从上面api可以看出，并没有直接传递字符串的方式。（期间用data、json、file-like各种试都不行）
但官方文档中有个说明：
There are many times that you want to send data that is not form-encoded. If you pass in a string instead of a dict, that data will be posted directly.
所以很简单，直接使用 data=str 就行。
res = requests.post(API_URL, data=url)
