Add python_requires corresponding to listed Python versions in setup.py

Add versioning for setuptools to support inclusiong of python_requires argument.
Note: This will require pip versions >=9.0.0 for support.

setup.py

@@ -62,14 +62,15 @@ def package_vars(version_file):
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
     ],
+    python_requires=">=3.6",
     # What does your project relate to?
     keywords="skeleton",
     packages=find_packages(where="src"),
     package_dir={"": "src"},
     package_data={"example": ["data/*.txt"]},
     py_modules=[splitext(basename(path))[0] for path in glob("src/*.py")],
     include_package_data=True,
-    install_requires=["docopt", "setuptools"],
+    install_requires=["docopt", "setuptools >= 24.2.0"],
     extras_require={
         "test": [
             "pre-commit",