gh-128317: Highlight today in colour in calendar CLI output

[hugovk]

Here's the macOS cal command:

Here's the calendar CLI output in 3.13:

And with this PR:

Especially useful for Twixtmas when you're not sure what day it is!

The usual PYTHON_COLORS, FORCE_COLOR and NO_COLOR env vars can be used to override terminal colour detection.

• Issue: Highlight today in colour in calendar's CLI output #128317

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📚 Documentation preview 📚: https://cpython-previews--128318.org.readthedocs.build/

[ZeroIntensity]

Nice, here's what it looks like on KDE :)

[hugovk]

Thanks all for the reviews!

[encukou]

Should this say “using terminal escape sequences”?

[hugovk]

Hmm, I think it's more useful to describe the end result rather than the "how".

[encukou]

That was my intention: mention that you need to print the result to a terminal to see the highlight. If you don't, you'll see some garbage bytes.