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

2 jobs for os-path-join in 3 minutes and 33 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #28729
pyFstat
pytest

00:03:25

 
  Static Analysis
failed #28730
pyFstat
static

00:00:07

 
Name Stage Failure
failed
static Static Analysis
Downloading artifacts for pytest (28729)...
Runtime platform arch=amd64 os=linux pid=3279 revision=05161b14 version=12.4.1
Downloading artifacts from coordinator... ok
id=28729 responseStatus=200 OK token=1z3WTS1A
$ source ${VENV_DIR}/bin/activate
$ black --check .
would reformat /home/gitlab-runner/builds/LCcssuc-/0/GregAshton/PyFstat/tests.py
All done! 💥 💔 💥
1 file would be reformatted, 27 files would be left unchanged.
ERROR: Job failed: exit status 1