Clean up ext_modules definition in setup.py

[duncanmmacleod]

This PR cleans up the definition of the extension module in setup.py. Most of the change is style, the only substantive change is to remove the unnecessary gslcblas link.

[mattpitkin]

From the build failure it looks like gslcblas is required.

[duncanmmacleod]

From the build failure it looks like gslcblas is required.

I think this is a linking issue in libgs23 for ubuntu-18.04, it clearly needs libgslclas itself but isn't linked against it. This is corrected on the ubuntu-20.04 image.