Add Colour.from_str factory method

This moves the command extension parsing code over to the main library
since it can be potentially useful for others.

tests/test_colour.py

@@ -0,0 +1,69 @@
+"""
+The MIT License (MIT)
+
+Copyright (c) 2015-present Rapptz
+
+Permission is hereby granted, free of charge, to any person obtaining a
+copy of this software and associated documentation files (the "Software"),
+to deal in the Software without restriction, including without limitation
+the rights to use, copy, modify, merge, publish, distribute, sublicense,
+and/or sell copies of the Software, and to permit persons to whom the
+Software is furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+DEALINGS IN THE SOFTWARE.
+"""
+
+from __future__ import annotations
+
+import discord
+import pytest
+
+
+@pytest.mark.parametrize(
+    ('value', 'expected'),
+    [
+        ('0xFF1294', 0xFF1294),
+        ('0xff1294', 0xFF1294),
+        ('0xFFF', 0xFFFFFF),
+        ('0xfff', 0xFFFFFF),
+        ('#abcdef', 0xABCDEF),
+        ('#ABCDEF', 0xABCDEF),
+        ('#ABC', 0xAABBCC),
+        ('#abc', 0xAABBCC),
+        ('rgb(68,36,59)', 0x44243B),
+        ('rgb(26.7%, 14.1%, 23.1%)', 0x44243B),
+        ('rgb(20%, 24%, 56%)', 0x333D8F),
+        ('rgb(20%, 23.9%, 56.1%)', 0x333D8F),
+        ('rgb(51, 61, 143)', 0x333D8F),
+    ],
+)
+def test_from_str(value, expected):
+    assert discord.Colour.from_str(value) == discord.Colour(expected)
+
+
+@pytest.mark.parametrize(
+    ('value'),
+    [
+        'not valid',
+        '0xYEAH',
+        '#YEAH',
+        '#yeah',
+        'yellow',
+        'rgb(-10, -20, -30)',
+        'rgb(30, -1, 60)',
+        'invalid(a, b, c)',
+        'rgb(',
+    ],
+)
+def test_from_str_failures(value):
+    with pytest.raises(ValueError):
+        discord.Colour.from_str(value)