[dependencies] Moved pillow to test requirements

Needed for matplotlib image output
7 jobs for dependencies_cleanup in 50 minutes and 20 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #175573
docker-build
build_linux_36

00:15:11

64.0%
passed #175563
docker-build
build_linux_37

00:15:49

64.0%
passed #175572
macosx
build_macosx_36

00:19:15

64.0%
passed #175561
macosx
build_macosx_37

00:15:46

64.0%
failed #175566
docker-build
build_linux_36

00:14:18

64.0%
failed #175562
docker-build
build_linux_36

00:15:19

64.0%
failed #175560
macosx
build_macosx_36

00:03:03