Default to true color rather than 256 colors
Default to true color rather than 256 colors

This brings Paint's actual behavior in line with its README.md file.
README.md states that Paint should default to 24bit color mode, i.e.,
Paint.detect_mode should fall back to returning Paint.TRUE_COLOR.

Previously, on operating systems other than Windows, Paint defaulted to
256 colors. With this commit it defaults to 24bit color on those
systems, even if the $TERM environment variable matches /-256color$/.
There is no reliable way to detect 24bit color support based on the
$TERM variable, and many terminals that support 24bit color will still
report as e.g. xterm-256color.

No changes were made to Paint's mode detection on Windows.

janlelis/paint