Make various os functions available on Windows (#6500)

stdlib/os/__init__.pyi

@@ -416,6 +416,13 @@ def getpid() -> int: ...
 def getppid() -> int: ...
 def strerror(__code: int) -> str: ...
 def umask(__mask: int) -> int: ...
+@final
+class uname_result(NamedTuple):
+    sysname: str
+    nodename: str
+    release: str
+    version: str
+    machine: str
 
 if sys.platform != "win32":
     def ctermid() -> str: ...
@@ -447,13 +454,6 @@ if sys.platform != "win32":
     def getsid(__pid: int) -> int: ...
     def setsid() -> None: ...
     def setuid(__uid: int) -> None: ...
-    @final
-    class uname_result(NamedTuple):
-        sysname: str
-        nodename: str
-        release: str
-        version: str
-        machine: str
     def uname() -> uname_result: ...
 
 @overload
@@ -469,7 +469,7 @@ if sys.platform != "win32":
 
 def putenv(__name: bytes | str, __value: bytes | str) -> None: ...
 
-if sys.platform != "win32":
+if sys.platform != "win32" or sys.version_info >= (3, 9):
     def unsetenv(__name: bytes | str) -> None: ...
 
 _Opener = Callable[[str, int], int]
@@ -563,7 +563,9 @@ else:
     def dup2(fd: int, fd2: int, inheritable: bool = ...) -> None: ...
 
 def fstat(fd: int) -> stat_result: ...
+def ftruncate(__fd: int, __length: int) -> None: ...
 def fsync(fd: FileDescriptorLike) -> None: ...
+def isatty(__fd: int) -> bool: ...
 def lseek(__fd: int, __position: int, __how: int) -> int: ...
 def open(path: StrOrBytesPath, flags: int, mode: int = ..., *, dir_fd: int | None = ...) -> int: ...
 def pipe() -> tuple[int, int]: ...
@@ -577,10 +579,8 @@ if sys.platform != "win32":
         def fdatasync(fd: FileDescriptorLike) -> None: ...  # Unix only, not Mac
     def fpathconf(__fd: int, __name: str | int) -> int: ...
     def fstatvfs(__fd: int) -> statvfs_result: ...
-    def ftruncate(__fd: int, __length: int) -> None: ...
     def get_blocking(__fd: int) -> bool: ...
     def set_blocking(__fd: int, __blocking: bool) -> None: ...
-    def isatty(__fd: int) -> bool: ...
     def lockf(__fd: int, __command: int, __length: int) -> None: ...
     def openpty() -> tuple[int, int]: ...  # some flavors of Unix
     if sys.platform != "darwin":