pfb-imaging master not building with 0.2.0

[o-smirnov]

Something with sympy. Will comment it out for now, but @landmanbester please take a look at #69.

[landmanbester]

This seems to happen after I added a more recent version of sympy to the requirements. My best guess is that one of my dependencies is probably installing an older version of sympy using distutils and pip doesn't want to uninstall and upgrade to a more recent (non distutils version) to prevent partial removal/incorrect installation. Relaxing the dependency seems to fix the build. Weirdly this does not happen for me locally. Anyway it should be fixed when ratt-ru/pfb-imaging#122 goes in

[o-smirnov]

Probably related to that setuptools "innovation" that's been biting @bennahugo and me.