Projects that use pyx.build (ex. wendelin.core) need recent
setuptools_dso fixes:

https://github.com/mdavidsaver/setuptools_dso/issues/5
https://github.com/mdavidsaver/setuptools_dso/commit/67d717a6
https://github.com/mdavidsaver/setuptools_dso/commit/e40f5883
https://github.com/mdavidsaver/setuptools_dso/commit/40b492ab

Increase setuptools_dso version in pygolang's build requirement for
uniformity as well.