Resolve "gevent exception (cannot switch to a different thread): SCAN_SAVING"

Closes #2715 (closed) #2711 (closed)

ptpython uses threads which is a problem for:

• the global Redis connection pools (only one thread can use those pools)
• the asyncio event loop which is built on top of gevent using aiogevent (can only be used in the main thread)

So we need to remove all usage of threads in case:

• Bliss code could be called
• The asyncio event loop is used

All places where threads are being used which may execute Bliss code:

PythonRepl.run_async:
 asyncio.get_event_loop().run_in_executor(None, ...)

PythonInput.read:
 threading.Thread

PythonInput._create_buffer:
 ThreadedCompleter
 ThreadedAutoSuggest

All places where threads are being used which do not execute Bliss code:

PythonInput._on_input_timeout:
 asyncio.get_event_loop().run_in_executor(None, ...)