tox: Cleanup
tox: Cleanup

Reduce indentation to 2 spaces, remove unused targets, and run
everything under Python 3 by default.

Signed-off-by: Stephen Finucane <06fa905d7f2aaced6dc72e9511c71a2a51e8aead@that.guru>

getpatchwork/git-pw,stephenfin/git-pw