[b4] Suggestion: make ezgb + git-bug optional dependencies

[Thomas Bracht Laumann Jespersen <laumann@gentoo.org>]

Hello,

I'd like to suggest that ezgb and git-bug become optional dependencies,
essentially like textual is an optional dependency today. The reason is
that by default "b4 bugs" prints:

    The TUI requires the textual library.
    Install it with: pip install b4[tui]

which gives me idea it's built around using the textual
dependency. git-bug also seems like a fairly large dependency for a
feature that doesn't really appear related to patch management in
public-inbox archives, and that it's about a different workflow entirely
(tracking bugs).

I took over packaging of b4 in Gentoo and found one unit test that
failed because it couldn't import ezgb._cli (this was
src/tests/test_packaging.py::test_public_modules_import_without_textual).
To fix this test, I've packaged git-bug and ezgb, and am about to update
the b4-0.16.0 package spec to require ezgb. In the process, I got the
idea that maybe it would make sense to have these dependencies be
optional.

All the best,
-- Thomas