fix style issues

setup.py

@@ -5,17 +5,8 @@
 
 import setuptools
 
-from packaging import version
 
 PACKAGE_NAME = 'responsibly'
-MINIMUM_PYTHON_VERSION = '3.6'
-
-
-def check_python_version():
-    """Exit when the Python version is too low."""
-    if (version.parse(sys.version.split()[0])
-        < version.parse(MINIMUM_PYTHON_VERSION)):
-        sys.exit("Python {0}+ is required.".format(MINIMUM_PYTHON_VERSION))
 
 
 def read_package_variable(key, filename='__init__.py'):
@@ -38,8 +29,6 @@ def build_description():
     return readme + '\n' + changelog
 
 
-check_python_version()
-
 setuptools.setup(
     name=read_package_variable('__project__'),
     version=read_package_variable('__version__'),
@@ -75,6 +64,8 @@ def build_description():
         'Programming Language :: Python :: 3.7',
     ],
 
+    python_requires='>=3.6',
+
     install_requires=[
         "numpy >= 1.15",
         "scipy >= 1.1",