Annual cleanup (#2159)

-  Simplify setup.py: remove unneeded features and get rid of deprecation warnings
-  Compatibility with Python 3.10: handle `threading.Event.isSet()` deprecation
-  Make sure setup.py could run without `six`: move Patroni class and main function to the `__main__.py`. The `__init__.py` will have only a few functions used by the Patroni class and from the setup.py

patroni.py

@@ -1,5 +1,5 @@
 #!/usr/bin/env python
-from patroni import main
+from patroni.__main__ import main
 
 
 if __name__ == '__main__':