Update git submodules

* Update requirements from branch 'master'
  - Merge "Lower Sphinx constraint"
  - Lower Sphinx constraint
    
    Sphinx 3.0.0 has introduced failures in several repos [0]. Until we can
    identify the cause/fix for these failures, and given the point in the
    release cycle, it might be best to keep this limited until we have some
    more time to understand the issue.
    
    [0] sphinx-doc/sphinx#7422
    
    Change-Id: I8cf2a0932701b7023eef22b5cb2b6d3088891389
    Signed-off-by: Sean McGinnis <sean.mcginnis@gmail.com>