lal.utils.series: fix style issues

as identified by flake8
31 jobs for lal-lint-errors in 217 minutes and 33 seconds (queued for 6 seconds)
Status Job ID Name Coverage
  .Pre
passed #598495
make-dist

00:06:26

 
  Lal
passed #598498
lal:conda:fftw

00:11:48

passed #598499
lal:conda:mkl

00:13:03

passed #598497
lal:deb

00:33:02

passed #598496
lal:rpm

00:19:19

 
  Lal Frame
passed #598502
lalframe:conda

00:08:09

passed #598501
lalframe:deb

00:08:39

passed #598500
lalframe:rpm

00:09:49

 
  Lal Meta Io
passed #598505
lalmetaio:conda

00:07:48

passed #598504
lalmetaio:deb

00:08:58

passed #598503
lalmetaio:rpm

00:08:56

 
  Lal Simulation
passed #598508
lalsimulation:conda

00:11:31

passed #598507
lalsimulation:deb

00:20:11

passed #598506
lalsimulation:rpm

00:12:09

 
  Lal Burst
passed #598511
lalburst:conda

00:09:01

passed #598510
lalburst:deb

00:19:33

passed #598509
lalburst:rpm

00:22:46

 
  Lal Inspiral
passed #598514
lalinspiral:conda

00:12:30

passed #598513
lalinspiral:deb

00:19:31

passed #598512
lalinspiral:rpm

00:14:07

 
  Lal Pulsar
passed #598517
lalpulsar:conda

00:13:09

passed #598516
lalpulsar:deb

00:22:12

passed #598515
lalpulsar:rpm

00:14:49

 
  Lal Inference
passed #598520
lalinference:conda

00:19:48

passed #598519
lalinference:deb

00:22:16

passed #598518
lalinference:rpm

00:18:55

 
  Lal Apps
passed #598523
lalapps:conda

00:18:12

passed #598522
lalapps:deb

00:18:38

passed #598521
lalapps:rpm

00:16:45

 
  Documentation
passed #598524
documentation

00:08:01

 
  Integration Tests
passed #598525
top-level

00:42:59