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 With outdated way, setuptools+pip fail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 for different versions (in our case,
Python 3.6 and Python 3.7 runs will fail with "No module named 'numpy'"
on pandas install while trying to install unsupported latest version).
Excessive build_py stage is also skipped.

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: