setup: update install command

Using `pip install .` instead of `python setup.py install` is a modern
way to install local package [1]. With outdated way, setuptools+pip fail
to resolve dependencies and CI pipelines for Python 3.6 and Python 3.7
fail with "No module named 'numpy'" on pandas install.

1. https://setuptools.pypa.io/en/latest/userguide/quickstart.html

Part of #238

Author: DifferentialOrange

Diff for: Makefile

@@ -1,6 +1,6 @@
 .PHONY: install test
 install:
-	python setup.py install
+	pip install --editable .
 test:
 	python setup.py test
 testdata: