Python 3.8: Replace deprecated set_process_end_time function

This MR fixes another python-ligo-lw compatibility issue associated with the upgrade from igwn-py37 (v1.7.x) to igwn-py38 (v1.8.3).

Issue Summary

As per this commit in python-ligo-lw, the ligo.lw.utils.process.set_process_end_time function has been deprecated in favor of a method on the Process class from the ligo.lw.lsctables module. The deprecated docstring states "Use .set_end_time_now() method of the Process object" which we now use.

The new .set_end_time_now() method can be found here: https://git.ligo.org/kipp.cannon/python-ligo-lw/-/blob/1.8.3/ligo/lw/lsctables.py#L605

After this change the run_py3_gw170817_offline_with_gout.sh script runs to completion with the following final terminal logs:

Note to Developers

@patrick.clearwater @timothyfrank.davies

The reason why this change as well as !85 (merged) is necessary is because in our installation of the pipeline on oz016 we are running igwn-py38 with python-ligo-lw==1.8.3, but the pipeline fails to run to completion without these changes.

However, these two (now deprecated) code snippets (.set_process_end_time and gz arguments which we have fixed here in !87 (merged) and in !85 (merged) respectively) in python-ligo-lw are still present in python-ligo-lw==1.7.x,which was the version set for the igwn-py37.

This must mean that whatever virtual environment the gstreamer_python_upgrade branch has been running on on oz996 must either have some installation or path issues with its python dependencies, or its actually just running the igwn-py37 conda environment rather than the expected igwn-py38 environment.