Require Notmuch 0.30+ for notmuch2 Python bindings

[tschwinge]

As of #173 "Migrate to notmuch2" etc., Lieer uses the notmuch2 Python bindings, which have first been shipped in Notmuch 0.30. (..., and 0.30 appears to still work fine for current Lieer, as I've just tried.)

[gauteh]

Thanks, should probably be added to setup.py and requirements.txt

[tschwinge]

So I looked into that -- and found out that https://pypi.org/project/notmuch2/ is actually (a) not based on the upstream Notmuch sources, (b) unmaintained (last changed four years ago), and (c) only offers a 0.30-rc2 pre-release from Jun 19, 2020. (I'm using my own local build of Notmuch.)

ERROR: Could not find a version that satisfies the requirement notmuch2>=0.30 (from versions: 0.1, 0.30rc2)
ERROR: No matching distribution found for notmuch2>=0.30

[gauteh]

Oh right. Yeah, I remember that was a mess. Let's merge as is then.