update readthedocs config

[lkeegan]

• add furo to docs requirements in pyproject.toml
• update .readthedocs.yaml
  • use Python 3.9
  • install rp-bp with pip install .[docs]

[codecov[bot]]

Codecov Report

Merging #148 (afc51a1) into dev-ssciwr (01bb10c) will not change coverage. The diff coverage is n/a.

@@             Coverage Diff             @@
##           dev-ssciwr     #148   +/-   ##
===========================================
  Coverage       48.68%   48.68%           
===========================================
  Files              34       34           
  Lines            4024     4024           
===========================================
  Hits             1959     1959           
  Misses           2065     2065           

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