fix: improve typing signature of .dropna()

Author: NickCrews

ibis/expr/types/relations.py

@@ -2064,7 +2064,9 @@ def count(self, where: ir.BooleanValue | None = None) -> ir.IntegerScalar:
         return ops.CountStar(self, where).to_expr().name("count")
 
     def dropna(
-        self, subset: Sequence[str] | None = None, how: Literal["any", "all"] = "any"
+        self,
+        subset: Sequence[str] | str | None = None,
+        how: Literal["any", "all"] = "any",
     ) -> Table:
         """Remove rows with null values from the table.