snglbursttable.py: fix import to _snglbursttable to not be relative, which...

snglbursttable.py: fix import to _snglbursttable to not be relative, which causes ValueError: Attempted relative import in non-package

gstlal-burst/python/snglbursttable.py

 from glue.ligolw import ilwd
 from glue.ligolw import lsctables
 import lal
-from . import _snglbursttable
+from gstlal import _snglbursttable
 
 
 __all__ = ["GSTLALSnglBurst"]