bpo-36675: Fix doctest directives comments disappear bug

[tczorro]

1. Change :: detective to : in order to fix the doc bugs
   ref:https://bugs.python.org/issue36675

https://bugs.python.org/issue36675

[the-knights-who-say-ni]

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[auvipy]

LGTM

[tczorro]

@JulienPalard I am sorry to bother you, but if you have some spare time, could you please review this little patch for doc fix.

[JulienPalard]

I tried a local build and your patch do not fix the issue. I'm investigating, I just found a trim_doctest_flags variable in tools/extensions/pyspecific.py, probably a good lead.

[JulienPalard]

I opened an issue here: sphinx-doc/sphinx#6698 and commented on b.p.o.

As this PR proposes a fix that does not fix the issue (the flags are still trimmed), I'll just close this one.