get_cost can return a float rather than an int

[Alex Rothberg]

shillelagh/src/shillelagh/adapters/base.py

Lines 105 to 109 in 14579e4

 def get_cost(  # pylint: disable=unused-argument, no-self-use 
     self, 
     filtered_columns: List[Tuple[str, Operator]], 
     order: List[Tuple[str, RequestedOrder]], 
 ) -> int: 

see the underlying apsw code: https://github.com/rogerbinns/apsw/blob/6913d52acbbb80b5a697d21817de744370ca72d2/src/vtable.c#L798-L804