test: add pytest and test config load/migrate (MR 2252)

re-introduce pytest, add a conftest.py with some useful fixtures and
basic tests for config loading.

This just checks that we can load the config and migrate it from the old
2.3.x format to the new 3.0 format with the new mirrors section.

Testing anything that requires args or Context should probably wait
until we can properly model state (since global state like in
get_context() really doesn't jive with pytest).

Signed-off-by: Caleb Connolly <caleb@postmarketos.org>