[setup] Fix matplotlib requirement

It's only useful for the tests as the documentation is built
during that phase.

Fixes #10
4 jobs for fix_matplotlib_dependency in 30 minutes and 50 seconds (queued for 4 seconds)
Status Name Job ID Coverage
  Build
passed build_linux_27 #126557
docker

00:28:54

90.0%
passed build_linux_36 #126558
docker

00:28:58

90.0%
passed build_macosx_27 #126559
macosx

00:30:50

90.0%
passed build_macosx_36 #126560
macosx

00:30:09

90.0%