mypy rejects _ from gettext.gettext.install()

[cfm]

This is a tracking ticket for the problem first identified in #1282 (comment), because it is out of scope of both the original ticket #1266 and its implementation in #1282.

There exists a triangular incompatibility among:

1. application-wide gettext configuration;
2. working locale fallback in securedrop_client.app.configure_locale_and_language(); and
3. mypy's fixed behavior with built-ins.

As currently implemented, #1282 can satisfy at most two of these three requirements at any given time:

Approach	(1)	(2)	(3)	Notes
Original implementation: gettext.gettext.install() + from gettext import gettext as _	✗	✓	✓	from gettext import gettext as _ in other modules effectively ignores the locale configured and translation loaded by gettext.gettext.install() in securedrop_client.app.
Implementation as of 4dd4d13: gettext.gettext.install()	✓	✓	✗	python/mypy#8727
Implementation with GNU gettext API	✓	✗	✓	gettext.[bind]textdomain will take $LANG from environment; does not support fallback configuration in configure_locale_and_language().

[cfm]

For the purposes of #1282, I will propose there a switch to the GNU gettext API. This will come at the cost of a feature that isn't actually implemented anyway: being able to switch languages within securedrop-client dynamically, regardless of the $LANG in the environment. It won't break the fallback gettext does automatically to the default locale (en) if the specified $LANG is not available.