No matter the combination of these USE flags - "python python_targets_python2_7 python_targets_python3_5 python_targets_python3_6" - net-wireless/soapysdr-0.7.0 will always build and install the Python bindings for 2.7 and the latest installed 3.x. A fixed ebuild, with separate build dirs for each Python version, is available in my overlay: https://github.com/stefantalpalaru/gentoo-overlay
Python2_7 no longer in tree. Actual package version respects python settings now. The link in #c 1 no longer points to an useable ebuild