Fix mypy: annotate drag_overlay_mode with full union type in Vertical and Tall

Bare literal assignments (drag_overlay_mode = 'axis_y') cause mypy to
narrow-infer the type as Literal['axis_y'] on the parent class, making
the subclass override (Horizontal = 'axis_x', Fat = 'axis_x') an
incompatible assignment. Fix by explicitly annotating Vertical and Tall
with the full union type from the base class, so the declared type stays
wide and subclasses can freely assign any valid mode.

Also removes unused ClassVar/Literal imports from splits.py.

kitty/layout/splits.py

@@ -2,7 +2,7 @@
 # License: GPLv3 Copyright: 2020, Kovid Goyal <kovid at kovidgoyal.net>
 
 from collections.abc import Collection, Generator, Iterator, Sequence
-from typing import Any, ClassVar, Literal, Optional, TypedDict, Union
+from typing import Any, Optional, TypedDict, Union
 
 from kitty.borders import BorderColor
 from kitty.fast_data_types import BOTTOM_EDGE, LEFT_EDGE, RIGHT_EDGE, TOP_EDGE

kitty/layout/tall.py

@@ -4,7 +4,7 @@
 import sys
 from collections.abc import Generator, Iterator, Sequence
 from itertools import islice, repeat
-from typing import Any
+from typing import Any, ClassVar, Literal
 
 from kitty.borders import BorderColor
 from kitty.conf.utils import to_bool
@@ -136,7 +136,7 @@ class Tall(Layout):
     name = 'tall'
     main_is_horizontal = True
     no_minimal_window_borders = True
-    drag_overlay_mode = 'axis_y'
+    drag_overlay_mode: ClassVar[Literal['full', 'axis_y', 'axis_x', 'free']] = 'axis_y'
     layout_opts = TallLayoutOpts({})
     main_axis_layout = Layout.xlayout
     perp_axis_layout = Layout.ylayout

kitty/layout/vertical.py

@@ -2,7 +2,7 @@
 # License: GPLv3 Copyright: 2020, Kovid Goyal <kovid at kovidgoyal.net>
 
 from collections.abc import Generator, Iterable
-from typing import Any
+from typing import Any, ClassVar, Literal
 
 from kitty.borders import BorderColor
 from kitty.types import Edges, NeighborsMap, WindowMapper
@@ -64,7 +64,7 @@ class Vertical(Layout):
     name = 'vertical'
     main_is_horizontal = False
     no_minimal_window_borders = True
-    drag_overlay_mode = 'axis_y'
+    drag_overlay_mode: ClassVar[Literal['full', 'axis_y', 'axis_x', 'free']] = 'axis_y'
     main_axis_layout = Layout.ylayout
     perp_axis_layout = Layout.xlayout