Re: Sympy(+sagemath+nipy+~30 more) removal ahead

["Rebecca N. Palmer" <rebecca_palmer@zoho.com>]

This involves a lot more than just sympy: the (build-)dependency chain is

execnet - sometimes FTBFS (3-4x unreproducible test failures, no known 
fix; disabling/ignoring the tests in question would probably make it 
build, but obviously isn't a real fix)
^
|
 pytest-xdist
^
|(apparently only for already-disabled tests)
|
wand <-- sorl-thumbnail willow
^
|
blockdiag - FTBFS (test mistakes annoying-but-harmless wand warning for 
an error) with patch
^
|(some indirectly)
|
actdiag brian cain chaussette dolfin fenics ffc fiat geophar ironic nipy 
nipype nova nwdiag octave-symbolic openstack-meta-packages 
python-measurement python-ws4py rabbit rabbiter sagemath seelablet 
seqdiag sfepy sphinxcontrib-actdiag sphinxcontrib-blockdiag 
sphinxcontrib-nwdiag sphinxcontrib-seqdiag sympy yt

Hence, we can save a lot more packages than just sympy by applying the 
blockdiag patch and either cutting wand->pytest-xdist or disabling 
execnet's problematic tests.