Reserved keyword used for kwarg causes SyntaxError on python>=3.7

The use of async as a keyword argument name causes a SyntaxError when attempting to build gstlal with python>=3.7:

  File "/home/duncan/opt/miniconda3/conda-bld/gstlal_1612180047180/_test_env_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_placehold_plac/lib/python3.8/site-packages/gstlal/pipeparts/__init__.py", line 402
    def mkmultifilesink(pipeline, src, next_file = 0, sync = False, async = False, **properties):
                                                                    ^
SyntaxError: invalid syntax

This needs to be renamed to something other than async. This is clearly seen on python3.6 with a DeprecationWarning:

$ python3.6 -c "async = test"
<string>:1: DeprecationWarning: 'async' and 'await' will become reserved keywords in Python 3.7
Traceback (most recent call last):
  File "<string>", line 1, in <module>
NameError: name 'test' is not defined