add python_requires to setup.py (#1123)

* add python_requires to setup.py so that unsupported python
interpreters don't install anyways

* Drop the upper-bound from python_requires
per #1123

setup.py

@@ -47,6 +47,7 @@
         'numba >= 0.43.0',
         'soundfile >= 0.9.0',
     ],
+    python_requires='>=3.6',
     extras_require={
         'docs': ['numpydoc', 'sphinx!=1.3.1', 'sphinx_rtd_theme',
                  'matplotlib >= 2.0.0',