Add html colors "grey" in addition to "gray"

[Aztah]

HTML supports both spellings "gray" and "grey" (https://www.w3schools.com/colors/colors_names.asp)

There are 7 different types of gray/grey but attempting to use any of the grey versions will raise:

TraitError: The 'value' trait of a ColorPicker instance expected a valid HTML color, not the str 'grey'.

The affected colors are: ['darkgrey', 'darkslategrey', 'dimgrey', 'grey', 'lightgrey', 'lightslategrey', 'slategrey']

For the ColorPicker widget the default behavior in case of a wrong color (eg. "asdf") is to do nothing, but inputting any of the greys will raise the above exception.

Adding the grey versions of to _color_names in trait_types.py will allow the grey names to go through the name check and render the same way as the gray counterpart.

Simple test code

import ipywidgets
import traitlets
import IPython

print(f'{ipywidgets.__version__ = }')
print(f'{traitlets.__version__ = }')
print(f'{IPython.__version__ = }')

grays = ['darkgray', 'darkslategray', 'dimgray', 'gray', 'lightgray', 'lightslategray', 'slategray']
greys = ['darkgrey', 'darkslategrey', 'dimgrey', 'grey', 'lightgrey', 'lightslategrey', 'slategrey']

for gray, grey in zip(grays, greys):
    try:
        display(ipywidgets.ColorPicker(value=gray))
    except ipywidgets.TraitError as e:
        print(e)
    try:
        display(ipywidgets.ColorPicker(value=grey))
    except traitlets.TraitError as e:
        print(e)

Output from current version:

Output from new version:

[github-actions]

👈 Launch a binder notebook on branch Aztah/ipywidgets/add_html_color_grey

[jasongrout]

Thanks!

[maartenbreddels]

meeseeksdev please backport to 7.x