Use correct typing.Literal for Python 3.7.

src/textual/widgets/_placeholder.py

@@ -1,13 +1,13 @@
 from __future__ import annotations
 
 from itertools import cycle
-from typing import Literal
 
 from .. import events
 from ..app import ComposeResult
 from ..css._error_tools import friendly_list
 from ..reactive import Reactive, reactive
 from ..widgets import Static
+from .._typing import Literal
 
 PlaceholderVariant = Literal["default", "size", "text"]
 _VALID_PLACEHOLDER_VARIANTS_ORDERED: list[PlaceholderVariant] = [