Providing context in translation strings requires Python 3.8

In !67 (merged) I tried to react to the issues in Transifex by adding context to some strings. This required using gettext.pgettext which is a new feature in Python 3.8.

Python 3.8 isn't available in Debian stable and it's still a year (?) until a new version of Debian is released.

With @grote I was discussing whether requiring Python 3.8 is worth only for providing context, because it excludes users on Debian stable.

Reading L10n Lab's Developer Guidelines I learned that there is an "Instructions field in the Web Editor to provide translators with context when possible". I don't seem to be able to use it since I'm only a Translator on Briar's project.

@akwizgran What's your opinion on that? Maybe we can also get Lab peoples' opinion on this?

Before reverting that changes and making yet another update to the source files, I wanted to hear your opinions on this topic.