fix regression in ExtensionCore not working when updates are available

[eli-schwartz]

In commit 6ac295a, Settings>Applets>Download gained the ability to display the number of available updates, using an ngettext-translated message. However, it has never worked since ngettext was never imported in the file in question, so it simply raised a traceback.

In the same commit, gettext.install() was taught to declare names="ngettext", but as per the python documentation, the names= argument requires a sequence (of one or more aliases to declare in the global namespace), not a string. So it tried to check if it could install the gettext functions: {'x', 't', 'e', 'g'}, which was wrong.

The fix is to either import ngettext directly, so it can be used as-is, or globally install ngettext by passing it as a one-item list to the gettext.install() function. I've done the latter.

[eli-schwartz]

ref: https://bugs.archlinux.org/task/64543

[clefebvre]

Already done in #8959.

[eli-schwartz]

I... don't even understand that other PR. It seems to be re-doing 058e3a1, fixing some gtk deprecation, and switching one incorrect use of gettext.install() for another, none of which has anything to do with the commit message (that claims this is for python 3.8).

[eli-schwartz]

@clefebvre

When the ngettext call was initially added in 6ac295a, I think the person who added it tried to use gettext.install(..., names="gettext"), but that's totally incorrect. Please reopen this PR. I can either leave it to continue explicitly importing ngettext from gettext (it works for me on python 3.8) or switch the gettext.install() call to use names=["gettext"] as it is required to be a sequence and not a string (well, a string is a sequence too, but you don't exactly get the right answer when you use set('ngettext') instead of set(['ngettext'])).

I initially used from gettext import ngettext, because I wasn't sure under what circumstances the gettext.install() was guaranteed to have run, BTW. But I'm totally okay with either solution.

[eli-schwartz]

I've switched to using gettext.install() with the correct sequence.