Merge branch 'fix/distutils' into 'master'

Mitigate setuptools warning

See merge request !18

qsymm/__init__.py

+from ._version import __version__
 from . import groups
 from . import linalg
 from . import hamiltonian_generator
@@ -13,4 +14,4 @@ from .symmetry_finder import symmetries, discrete_symmetries, conserved_quantiti
                              continuous_symmetries, bravais_point_group
 from .kwant_continuum import sympify
 
-from ._version import __version__
+del _version

qsymm/_version.py

@@ -4,9 +4,8 @@
 from collections import namedtuple
 import os
 import subprocess
-import sys
 
-from distutils.command.build_py import build_py as build_py_orig
+from setuptools.command.build_py import build_py as build_py_orig
 from setuptools.command.sdist import sdist as sdist_orig
 
 Version = namedtuple('Version', ('release', 'dev', 'labels'))