Fix some mypy typing issues

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

colorhash/cli.py

@@ -8,7 +8,7 @@ import textwrap
 from .colorizer import PaletteColorizer
 from .matricizer import Matricizer, NibbleMatricizer, RandomartMatricizer
 from .palettes import Palette, DEFAULT_PALETTES, PALETTES
-from .writer import ANSIWriter, SVGWriter
+from .writer import ANSIWriter, SVGWriter, Writer
 
 
 # TODO - WASM compile for embedding directly in HTML

colorhash/color.py

@@ -10,6 +10,7 @@ class Color(metaclass=abc.ABCMeta):
     This can be used to convert any color format into another color format.
     """
 
+    @abc.abstractmethod
     def to_html_color(self) -> str:
         """
         Convert this color to an HTML color.

colorhash/palettes.py

@@ -6,7 +6,7 @@ from .color import Color, HSLColor
 
 
 HSLRange = range | float | int | list[float | int]
-Palette = Sequence[str]
+Palette = Sequence[Color]
 
 
 def quantize(r: range, steps: int = 16) -> list[float]: