dev-lang/python: Set PYTHON_SITEDIR manually

The eclass change in fd4001e9 causes python_export to call ${PYTHON}
to obtain PYTHON_SITEDIR. This returns an empty string if ${PYTHON}
has not yet been merged.

Package-Manager: portage-2.2.23_p26

dev-lang/python/python-3.5.0-r2.ebuild

@@ -277,7 +277,8 @@ src_install() {
 		-i "${ED}etc/conf.d/pydoc-${PYVER}" "${ED}etc/init.d/pydoc-${PYVER}" || die "sed failed"
 
 	# for python-exec
-	python_export python${PYVER} EPYTHON PYTHON PYTHON_SITEDIR
+	python_export python${PYVER} EPYTHON PYTHON
+	export PYTHON_SITEDIR="${EPREFIX}/usr/$(get_libdir)/python${PYVER}/site-packages"
 
 	# if not using a cross-compiler, use the fresh binary
 	if ! tc-is-cross-compiler; then