use os.path.join instead of manual string joining with "/"

2 jobs for os-path-join in 3 minutes and 41 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #28727
pyFstat
pytest

00:03:25

 
  Static Analysis
failed #28728
pyFstat
static

00:00:15

 
Name Stage Failure
failed
static Static Analysis
would reformat /home/gitlab-runner/builds/LCcssuc-/0/GregAshton/PyFstat/pyfstat/core.py
would reformat /home/gitlab-runner/builds/LCcssuc-/0/GregAshton/PyFstat/pyfstat/grid_based_searches.py
would reformat /home/gitlab-runner/builds/LCcssuc-/0/GregAshton/PyFstat/pyfstat/helper_functions.py
would reformat /home/gitlab-runner/builds/LCcssuc-/0/GregAshton/PyFstat/pyfstat/make_sfts.py
would reformat /home/gitlab-runner/builds/LCcssuc-/0/GregAshton/PyFstat/tests.py
would reformat /home/gitlab-runner/builds/LCcssuc-/0/GregAshton/PyFstat/pyfstat/mcmc_based_searches.py
All done! 💥 💔 💥
6 files would be reformatted, 22 files would be left unchanged.
ERROR: Job failed: exit status 1