py-numpy build error due to conflict with enum34

[RJVB (René Bertin)]

py-numpy fails to build when the enum34 package is installed; see ​https://github.com/iterative/dvc/issues/1995 for symptoms and description.

This is a tricky one because there is no enum34 port for current python3 versions. Port:py27-numpy can register a build conflict on port:py27-enum34 but the python3 subports would have to do a custom check in the pre-configure or pre-build phase. The error is non-specific enought that this would IMHO be the courteous thing to do, even if users shouldn't install foreign stuff into $prefix (but if you use port:pythonXY as your main Python provide you'll probably end up doing that anyway; I had enum34 installed on 2 independent systems).