dev-python/setuptools_scm: Install package for testing

The test suite fails unless the package is already installed.

Bug: https://bugs.gentoo.org/show_bug.cgi?id=609648

dev-python/setuptools_scm/setuptools_scm-1.15.0.ebuild

@@ -26,5 +26,6 @@ DEPEND="
 	)"
 
 python_test() {
+	distutils_install_for_testing
 	py.test -v -v -x || die "tests failed under ${EPYTHON}"
 }