Improve legibility on light-background terminals

[dlitz]

This fixes the problem of light-gray text being rendered on a light background.

Users can now choose a different Pygments style/theme using the PY_DEVTOOLS_STYLE environment variable.

If PY_DEVTOOLS_STYLE is not set, we attempt auto-detection using the COLORFGBG environment variable, as is commonly done by some tools on Linux and macOS.

• Dark-background terminals use the "vim" style, as before.
• Light-background terminals now use the "sas" style.
• If auto-detection fails, we use the "default" style, which is legible on most terminals.

A list of available styles can be obtained by running:

import pygments.styles
list(pygments.styles.get_all_styles())

Screenshots

Light background (COLORFGBG=0;15)

Before	After

Dark background (COLORFGBG=15;0)

Before	After

Indeterminate (COLORFGBG missing or unparsable)

Dark Background	Light Background

[dlitz]

Fixed the commit message & CI builds.