Issue #11647: allow contextmanager objects to be used as decorators as described in the docs. Initial patch by Ysj Ray.

interpreter is shutting down; instead, these threads are now killed when
they try to take the GIL.

interpreter is shutting down; instead, these threads are now killed when
they try to take the GIL.

The U flag is no more used (but still accepted for backward compatibility).

Update the doc. Refactor also related tests.

The _tkinter module loads the Tcl library which creates a thread waiting events
in select(). This thread receives signals blocked by all other threads. We
cannot test blocked signals if the _tkinter module is loaded.