abspaths slipping into SOURCES and breaking windows conda build

[richardjgowers]

conda-forge/mdanalysis-feedstock#28

^^ On the 1.0.1 builds Windows is failing because some of the sources in the sdist are given as absolute paths.

This is a good description:

https://github.com/pyFFTW/pyFFTW/pull/250/files

It looks like we started trying to use abspath to get RTD to work here:

#2749

probably because of our slightly unusual repo structure

[IAlibay]

Fixed with #3130 - will need forward porting