python: Fix exception handler compatibility.

Python 3 dropped exception handlers of the deprecated form:

  except Exception, e:

You must use the newer syntax of:

  except Exception as e:

This patch also enables a flake8 warning for this.

  H231 Python 3.x incompatible 'except x,y:' construct

Signed-off-by: Russell Bryant <russell@ovn.org>
Acked-by: Ben Pfaff <blp@ovn.org>