Add classifer details to setup.py including Matplotlib framework

[hectormz]

Description

• This PR adds classifier details to setup.py, which will populate details about the project on PyPI
• This was inspired by Matplotlib receiving a trove classifier for its framework (Request trove classifier: matplotlib pypi/support#232), and wanting to add it Arviz
• I also updated the README to reflect the support for 3.5 (on CI) has been dropped in favor of 3.8
• This PR resolves Add Matplotlib framework classifier to setup.py #1108

Checklist

• Follows official PR format
• Code style correct (follows pylint and black guidelines)
• Changes are listed in changelog

[codecov]

Codecov Report

Merging #1133 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1133   +/-   ##
=======================================
  Coverage   92.68%   92.68%           
=======================================
  Files          93       93           
  Lines        9073     9073           
=======================================
  Hits         8409     8409           
  Misses        664      664           

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 9909aab...37cbbc2. Read the comment docs.

[ahartikainen]

LGTM

[OriolAbril]

LGTM too

[aloctavodia]

Thanks @hectormz!