python: convert shebangs to python3

This is the last bit of "python" left in the notmuch codebase.

https://www.python.org/dev/peps/pep-0394/#recommendation encourages
"third-party distributors" to use more-specific shebang lines.  I'm
not certain that the notmuch project itself is a "third-party
contributor" but I think this is a safe way to encourage people to use
python3 when they're developing notmuch.

We already have python3 explicitly elsewhere in the codebase for
developers (in nmbug).

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>