bijection kwargs in __init__ aren't typed

[mlenzen]

collections_extended/bijection.py:38: error: Invalid index type "str" for "bijection[K, V]"; expected type "K"

Keyword args to bijection.__init__ are always of type str and this may conflict with the declared type. How to handle? Could we raise a mypy error if kwargs are used and K is not str or a subtype?