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 minutes and 43 seconds)
latest
Status Job ID Name Coverage
  Test
passed #28737
pyFstat
pytest

00:03:24

 
  Static Analysis
failed #28738
pyFstat
static

00:00:08

 
Name Stage Failure
failed
static Static Analysis
         )
FS = search_H1L1.get_fullycoherent_twoF(
Writer.tstart,
Writer.tend,
Writer.F0,
would reformat tests.py
All done! 💥 💔 💥
1 file would be reformatted, 27 files would be left unchanged.
ERROR: Job failed: exit status 1