Drop distutils.core from setup.py

[MaxGhenis]

I think the below lines can be replaced with import setuptools now that taxcalc is Python 3+ only. Is this right?

Tax-Calculator/setup.py

Lines 1 to 5 in f3f7d95

	try:
	from setuptools import setup
	except ImportError:
	from distutils.core import setup

Based on Flask documentation:

If you have Python 2 (>=2.7.9) or Python 3 (>=3.4) installed from python.org, you will already have pip and setuptools on your system.

[jdebacker]

Closed via PR #2592.