Stop using deprecated setuptools method of install

The new way to install our Python3 interfaces is:

    python3 -m pip install -e .

See https://docs.python.org/3.6/installing/index.html#installing-index

agent/Makefile

@@ -253,8 +253,8 @@ install-lib:
 install-python3-setup: install-util-scripts install-lib
 	${COPY} requirements.txt ${DESTDIR}
 	mkdir -p ${DESTDIR}/python3
-	(cd ..; SKIP_GENERATE_AUTHORS=1 SKIP_WRITE_GIT_CHANGELOG=1 python3 setup.py install --prefix=${DESTDIR}/python3)
-	${COPY} $(addprefix ${DESTDIR}/python3/local/bin/, ${click-scripts}) ${UTILDIR}/
+	(cd ..; python3 -m pip install --prefix=${DESTDIR}/python3 -e .)
+	${COPY} $(addprefix ${DESTDIR}/python3/bin/, ${click-scripts}) ${UTILDIR}/
 	rm -rf ${DESTDIR}/python3
 	${COPY} ../lib/pbench ${LIBDIR}/
 	rm -r $$(find ${LIBDIR} -name __pycache__) ${LIBDIR}/pbench/test ${LIBDIR}/pbench/server ${LIBDIR}/pbench/cli/server