require mutagen >= 1.20

This release fixes several bugs, one of which influences the test cases of
beets (in particular, the MusePack bitrate calculation).
towards filesystem sandboxing of tests

The generic test harness now uses a temporary directory for beets' various
files as well as $HOME. As one packager pointed out, there were various test
failures when $HOME did not exist. This is no longer the case.
tests: remove ad-hoc temporary directories

Now that we have a temporary directory in the shared harness, we can dispense
with the error-prone process of creating them for each test.
test: move DummyIO initialization to harness

This makes it much harder to accidentally leave this I/O isolator installed.