Use Literal for statistics.quantiles() (#7463)

I'm going to merge; I think this is consistent with what we've done with e.g. the `byteorder` argument for `int.from_bytes` :)

stdlib/statistics.pyi

@@ -3,6 +3,7 @@ from _typeshed import Self, SupportsRichComparisonT
 from decimal import Decimal
 from fractions import Fraction
 from typing import Any, Hashable, Iterable, NamedTuple, Sequence, SupportsFloat, TypeVar, Union
+from typing_extensions import Literal
 
 if sys.version_info >= (3, 10):
     __all__ = [
@@ -102,7 +103,9 @@ def pstdev(data: Iterable[_NumberT], mu: _NumberT | None = ...) -> _NumberT: ...
 def pvariance(data: Iterable[_NumberT], mu: _NumberT | None = ...) -> _NumberT: ...
 
 if sys.version_info >= (3, 8):
-    def quantiles(data: Iterable[_NumberT], *, n: int = ..., method: str = ...) -> list[_NumberT]: ...
+    def quantiles(
+        data: Iterable[_NumberT], *, n: int = ..., method: Literal["inclusive", "exclusive"] = ...
+    ) -> list[_NumberT]: ...
 
 def stdev(data: Iterable[_NumberT], xbar: _NumberT | None = ...) -> _NumberT: ...
 def variance(data: Iterable[_NumberT], xbar: _NumberT | None = ...) -> _NumberT: ...