follow-up https://github.com/spesmilo/electrum/pull/7202
defaultdict[int] is a type!
```
>>> from collections import defaultdict
>>> d = defaultdict[int]
>>> d[2]
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: There are no type variables left in collections.defaultdict[int]
```
Also, prior to py3.9, it is a TypeError.
7354feeffe