Add Python 3.10 classifier

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Author: pylipp

setup.py

@@ -26,6 +26,7 @@
  'Programming Language :: Python :: 3.7',
  'Programming Language :: Python :: 3.8',
  'Programming Language :: Python :: 3.9',
+ 'Programming Language :: Python :: 3.10',
  'Programming Language :: Python :: 3 :: Only',
  'Programming Language :: Python :: Implementation :: CPython',
  'Programming Language :: Python :: Implementation :: PyPy',