SetupTools 58 dropped support for its old use_2to3 option, which has
started surfacing a number of ancient Python packages in need of
updates. In this case, the last full release of funcparserlib (which
is a transitive dependency by way of blockdiag by way of
sphinxcontrib-blockdiag) was in 2013, but luckily they have an alpha
release which we can pin explicitly and pull in as a temporary
workaround to get docs builds going again.
Change-Id: I6903eeac2c479e2da795c1dbd215cdee33d09fd7
f8a60d416f