fix plot_pair labels

[OriolAbril]

Description

Fixes #1531. This PR makes only a couple minor changes to plot_pair. The first one is to
fix the generation of the plot labels and the second is to avoid calling filter twice. As
plot_pair generates a half grid of plots, its interpretation of max_subplots is not done on
the number of variables like with most other plots but on the actual number of subplots, may not be
ideal but the filtering is currently implemented at the backend level.

Checklist

• Does the PR follow official
  PR format?
• Is the code style correct (follows pylint and black guidelines)?
• Is the fix listed in the Maintenance and fixes
  section of the changelog?

[michaelosthege]

The error in the failing job does not make any sense to me https://dev.azure.com/ArviZ/ArviZ/_build/results?buildId=3851&view=logs&j=af9a1d3a-ff8f-5483-6823-86518c773a9b&t=d1f9f1df-bd17-5f75-5a7b-8e272c593e8b&l=83

[OriolAbril]

The error in the failing job does not make any sense to me https://dev.azure.com/ArviZ/ArviZ/_build/results?buildId=3851&view=logs&j=af9a1d3a-ff8f-5483-6823-86518c773a9b&t=d1f9f1df-bd17-5f75-5a7b-8e272c593e8b&l=83

Yeah, there is something going on with pyjags, we have disabled all its test temporally. I expect everything to work now and will make a patch release.