Add type hints for the zoneinfo module (#4038)

This is the implementation for PEP 615: https://www.python.org/dev/peps/pep-0615/

It is present starting in 3.9.0 beta 1.

stdlib/3.9/zoneinfo/__init__.pyi

@@ -0,0 +1,45 @@
+import os
+import typing
+from datetime import datetime, tzinfo
+from typing import (
+    Any,
+    Iterable,
+    Optional,
+    Protocol,
+    Sequence,
+    Set,
+    Type,
+    Union,
+)
+
+_T = typing.TypeVar("_T", bound="ZoneInfo")
+
+class _IOBytes(Protocol):
+    def read(self, __size: int) -> bytes: ...
+    def seek(self, __size: int, __whence: int = ...) -> Any: ...
+
+class ZoneInfo(tzinfo):
+    @property
+    def key(self) -> str: ...
+    def __init__(self, key: str) -> None: ...
+    @classmethod
+    def no_cache(cls: Type[_T], key: str) -> _T: ...
+    @classmethod
+    def from_file(
+        cls: Type[_T], __fobj: _IOBytes, key: Optional[str] = ...
+    ) -> _T: ...
+    @classmethod
+    def clear_cache(cls, *, only_keys: Iterable[str] = ...) -> None: ...
+
+# Note: Both here and in clear_cache, the types allow the use of `str` where
+# a sequence of strings is required. This should be remedied if a solution
+# to this typing bug is found: https://github.com/python/typing/issues/256
+def reset_tzpath(
+    to: Optional[Sequence[Union[os.PathLike, str]]] = ...
+) -> None: ...
+def available_timezones() -> Set[str]: ...
+
+TZPATH: Sequence[str]
+
+class ZoneInfoNotFoundError(KeyError): ...
+class InvalidTZPathWarning(RuntimeWarning): ...