New deb source arpy dependency is too hard

Sources by themselves are not supposed to expand on BuildStream's hard dependencies, we mostly use preflight for this.

Two things need to be done for this:

• The deb source plugin must not import arpy at the toplevel

  • It should try / except the import at preflight() time and report a sensible SourceError
  • And import arpy separately in the one function where it's used.

• The test suite needs to also conditionalize this, so that ./setup.py test does not bail out in the absence of arpy

  • Add HAVE_ARPY variable in the usual way to tests/testutils/site.py
  • Conditionalize the tests which use arpy in the usual way
  • Because the import statement is what causes test to bail, the import arpy statement in the test case probably needs to be moved into the individual test functions, too