python3Packages.dbf: fix build

Broken in 63155af44b. Failed with:

> Traceback (most recent call last):
>   File "/build/dbf-0.99.9/dbf/./test.py", line 15, in <module>
>     from . import *
> ImportError: attempted relative import with no known parent package

Relative imports only work when running as a module, not directly as a
file.

pkgs/development/python-modules/dbf/default.nix

@@ -23,7 +23,7 @@ buildPythonPackage rec {
   ];
 
   checkPhase = ''
-    ${python.interpreter} dbf/test.py
+    ${python.interpreter} -m dbf.test
   '';
 
   pythonImportsCheck = [