Declare support for Python 3.8-3.10

Author: hugovk

setup.py

@@ -30,6 +30,9 @@ def main():
                      'Programming Language :: Python :: 3.5',
                      'Programming Language :: Python :: 3.6',
                      'Programming Language :: Python :: 3.7',
+                     'Programming Language :: Python :: 3.8',
+                     'Programming Language :: Python :: 3.9',
+                     'Programming Language :: Python :: 3.10',
                      'Programming Language :: Python :: Implementation :: CPython',
                      'Programming Language :: Python :: Implementation :: PyPy',
                     ],