Have "make check" to do nothing for Python 3

In preparation for Python 3, don't run any tests for make check.
We'll enable individual tests once bup supports Python 3 well enough
for them to pass.

This shouldn't risk producing a false sense of security because you
still have to set BUP_ALLOW_UNEXPECTED_PYTHON_VERSION=true or bup
won't run at all, and if you've seen the warning produced when you
don't set it, you should be sufficiently well informed.

Signed-off-by: Rob Browning <rlb@defaultvalue.org>
Tested-by: Rob Browning <rlb@defaultvalue.org>