asyncio: test_selector_events: remove duplicate call to _on_handshake() method

The _SelectorSslTransport constructor already calls it.
Issue #21163, asyncio: Ignore "destroy pending task" warnings for private tasks
in gather().
Issue #21163: Fix "destroy pending task" warning in test_wait_errors()
asyncio: test_as_completed(): disable "slow callback" warning
Remove Tools/scripts/pydocgui.pyw.

pydoc.gui was removed over 3 years ago (#10818), this script has been
utterly bereft of worth since then.