Fix drop_first method name for typing

bf1a74d
added suport for "drop_front" but internally named this method "drop_first".

Since the test code uses "drop_first", and to prevent creating a backwards
incompatible change, I have chosen to fix the type defintion rather than the
internal method name, even though the origin commit used "drop_front" in the
commit message.

Author: byron-lambda

Diff for: rpds.pyi

@@ -49,4 +49,4 @@ class List(Iterable[T]):
     def __iter__(self) -> Iterator[T]: ...
     def __len__(self) -> int: ...
     def push_front(self, value: T) -> "List[T]": ...
-    def drop_front(self) -> "List[T]": ...
+    def drop_first(self) -> "List[T]": ...