I guess key should be bytes instead of a dict with string key. Maybe the docs could be improved about the case where key in case of dict should be bytes.

>>> from urllib.request import urlopen, Request
>>> r = Request("http://httpbin.org/post", data={b'a': 'a'})
>>> resp = urlopen(r)
>>> print(resp.read())
b'{\n  "args": {}, \n  "data": "", \n  "files": {}, \n  "form": {\n    "a": ""\n  }, \n  "headers": {\n    "Accept-Encoding": "identity", \n    "Content-Length": "1", \n    "Content-Type": "application/x-www-form-urlencoded", \n    "Host": "httpbin.org", \n    "User-Agent": "Python-urllib/3.8"\n  }, \n  "json": null, \n  "origin": "123.201.136.26, 123.201.136.26", \n  "url": "https://httpbin.org/post"\n}\n'

See https://bugs.python.org/issue25439 for an open patch to ensure this is type checked.

>>> from urllib.request import urlopen, Request
>>> r = Request("http://httpbin.org/post", data={'a': 'a'})
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/urllib/request.py", line 332, in __init__
    self.data = data
  File "/Users/karthikeyansingaravelan/stuff/python/cpython/Lib/urllib/request.py", line 370, in data
    raise TypeError("Key should be type of bytes in POST data")
TypeError: Key should be type of bytes in POST data

Current code is not checking the type makes it pass during construction and then throw an error later like below : 

>>> from urllib.request import urlopen, Request
>>> r = Request("http://httpbin.org/post", data={'a': 'a'})
>>> urlopen(r)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/urllib/request.py", line 222, in urlopen
    return opener.open(url, data, timeout)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/urllib/request.py", line 525, in open
    response = self._open(req, data)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/urllib/request.py", line 543, in _open
    '_open', req)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/urllib/request.py", line 503, in _call_chain
    result = func(*args)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/urllib/request.py", line 1345, in http_open
    return self.do_open(http.client.HTTPConnection, req)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/urllib/request.py", line 1317, in do_open
    encode_chunked=req.has_header('Transfer-encoding'))
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/http/client.py", line 1229, in request
    self._send_request(method, url, body, headers, encode_chunked)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/http/client.py", line 1275, in _send_request
    self.endheaders(body, encode_chunked=encode_chunked)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/http/client.py", line 1224, in endheaders
    self._send_output(message_body, encode_chunked=encode_chunked)
  File "/Library/Frameworks/Python.framework/Versions/3.7/lib/python3.7/http/client.py", line 1054, in _send_output
    + b'\r\n'
TypeError: can't concat str to bytes