Remove restriction on Python 3.6

Author: singularitti

setup.py

@@ -40,8 +40,10 @@ def find_version(*file_paths):
           'Topic :: Scientific/Engineering :: Physics',
           'License :: OSI Approved :: GNU General Public License v3 (GPLv3)',
           'Programming Language :: Python :: 3.6',
+          'Programming Language :: Python :: 3.7',
+          'Programming Language :: Python :: 3.8',
       ],
-      python_requires='>=3.6, <3.7',
+      python_requires='>=3.6',
       keywords='thermodynamic-properties quasi-harmonic-approximation scientific-computation',
       install_requires=[
           'lazy_property',