In commit 4a653e6 ("backends/winrt: raise exception when trying to scan
with STA") we added a check to raise an exception when trying to scan
when PyWinRT set the aparatment model to STA. However, properly working
GUI apps will have the apartment model set to STA but Bleak will still
work because there is something pumping the Windows message loop.

We don't want to raise an exception in this case to avoid breaking
working apps. We can improve the test by checking if the current thread
is actually pumping the message loop by scheduling a callback via a
the win32 SetTimeout function. If the callback is called, then we know
that the message loop is being pumped. If not, then we probably are not
going to get async callbacks from the WinRT APIs and we raise an
exception in this case.