Add curses.has_extended_color_support() for Python 3.10 (#5549)

stdlib/_curses.pyi

@@ -291,6 +291,10 @@ def getsyx() -> Tuple[int, int]: ...
 def getwin(__file: BinaryIO) -> _CursesWindow: ...
 def halfdelay(__tenths: int) -> None: ...
 def has_colors() -> bool: ...
+
+if sys.version_info >= (3, 10):
+    def has_extended_color_support() -> bool: ...
+
 def has_ic() -> bool: ...
 def has_il() -> bool: ...
 def has_key(__key: int) -> bool: ...