py3 build: circumvent non-conforming suffix in library names by numpy distutils
library names avoid hardcoded values now and should be only dependent on
what Python's builtin `sysconfig` says (that's what numpy distutils uses
during building internally, too) see #771
refactor 827ae127f88cc39d1, depending on how we import the extensions
(`ctpyes`/`__import__`) we sometimes have to leave away the extension
suffix (see taup). This should be clearer now. Fixes #771.