Odd error message when kwargs passed as **dict

Checking this with mypy 0.620:

from concurrent.futures import ThreadPoolExecutor
import sys

opts = {'max_workers': None}
if sys.version_info >= (3, 6):
    opts['thread_name_prefix'] = 'SyncWorker'
executor = ThreadPoolExecutor(**opts)

...yields:

mypy_bug.py:6: error: Incompatible types in assignment (expression has type "str", target has type "None")
mypy_bug.py:7: error: Argument 1 to "ThreadPoolExecutor" has incompatible type "**Dict[str, None]"; expected "str"

Let's ignore the first error for now. But the second seems to have an incorrect message, specifically I think the **Dict[str, None] in it is weird. Just guessing, is it trying to assign something to the ThreadPoolExecutor's thread_name_prefix arg which is a str? Maybe the None from max_workers because passing kwargs as a dict like this might result in random/varying order of the dict items? But if that's the case, I'd expect it to say something else than **Dict[str, None] as the culprit, maybe None or int, but not the whole **dict.

Another thought is that when kwargs is passed as **dicts like this, their order is not really defined, so "Argument 1" isn't very helpful. Would be better to say the argument's name.

But maybe I'm just all off track here -- will leave the rest to someone who actually has an informed opinion :)

issue の ThreadPoolExecutor 再現コードから始め、mypy 0.620 に対して実行して、**opts に関して報告された診断を調べます。辞書ベースのキーワード引数の処理を確認し、エラーが何を特定すべきかを判断します。診断が正確で、関連する引数または値を示していれば完了です。