[python-control-developers] [python-control/python-control] 35f1e6: Sphinx update (#479)

[Richard M. <no...@gi...>]

  Branch: refs/heads/master
  Home:   https://github.com/python-control/python-control
  Commit: 35f1e6486004bd35e2fcad399572754dd609d979
      https://github.com/python-control/python-control/commit/35f1e6486004bd35e2fcad399572754dd609d979
  Author: Richard Murray <mu...@cd...>
  Date:   2020-12-28 (Mon, 28 Dec 2020)

  Changed paths:
    M control/modelsimp.py
    M doc-requirements.txt

  Log Message:
  -----------
  Sphinx update (#479)

* fix footnote reference in markov() docstring

* require sphinx 3.4 or higher for readthedocs