Can you add a reference for the coefficients?

I believe

def test_main():
 test.support.run_unittest(ColorsysTest)

if __name__ == "__main__":
 test_main

has been and is being replaced in other test files with

if __name__ == "__main__":
 unittest.main

and should be here.

This should get a short What's New entry in the library section, something like

colorsys:
"The number of digits in the coefficients for the RGB -- YIQ conversions have been expanded so that they match the FCC NTSC versions. The change in results should be less than 1% and may better match results found elsewhere."

(You claim about the current rounding is not exactly correct. While .28*g rounds .277 rather than .274, the current .52*g rounds the non-FCC .523 rather than the FCC .5251. So I avoided making the claim in the suggested entry. It is not important.)