fix use of file() in ligolw_segments

- and bump version to 1.7.1
5 jobs for 1.7.1 in 2 minutes and 53 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Build
passed build-python27 #1203365

00:01:49

passed build-python37 #1203366

00:01:29

 
  Docs
passed docs #1203367

00:00:41

 
  Deploy
passed pages #1203368

00:00:22

passed pages:deploy

00:00:00