removed setup.py, fixed scons dev in virtual environments

Pip (or other build frontends) will now exclusively use setuptools with
setup.cfg instead of setup.py.

Recent versions of pip refuse to install packages outside of virtual
environements, and scons dev would previously force editable installation to
~/.local, causing confusion when using virtual envs. Now scons dev should only
work in virtual envs by default. To install and editable package outside of a
virtual env, use the PIPFLAGS option (not recommended).