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 With outdated way, setuptools+pip fail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 while trying to
install unsupported latest version.

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: install
 	python setup.py test
 testdata: