text: Title: Zombie excel instances from RunPython [pywin32 > 301 affected]
  Body: #### OS: Windows 10
  #### Versions: xlwings 0.25, Excel 365, Python 3.9
  
  ## Setup
  I've build a custom excel ribbon add-in, which calls different subs on click. I am not using the autogenerated functions as I don't have access to the vba model. Thus i've written simple RunPython calls in vba which call the python functions (no sub decorator needed). I am using the optimized connection, i.e. the server does not shut down after a single call.
  
  ## Problem
   If i have any reference to the xw.app in my sub, i.e. even a simple assignment like 
  ```python
  def my_first_sub():
         a = xw.book.caller().app
  ```
  or
  ```python
  def my_second_sub():
         a = xw.Book()
  ```
  the python process is not cleaned up after closing all Excel windows. Even worse, the gobal COM reference (?) to the app/book prevents a proper shutdown of Excel, which results in an "invisible" zombie process. If excel is started again, all add-ins are missing and overall it is broken until killed by i.e. the task manager. 
  
  ## Solution (?)
  1) One of my use cases is using the status_bar property of app.api or a win32 message box using the app.api.hwnd for i.e. logging. In order to prevent zombie I start another COM client Dispatcher (is that even a term?) inside the function and set the StatusBar property "without" xlwings.
  
  2) My second use case requires usage of xlwings' methods, so a "manual" COM connection is no alternative. I solved this so far by using a separate thread, i.e. something along these lines:
  
  ```python
  def do_something():
      a  = xw.Book()
     # Do something
  
  def my_second_sub():
         t = threading.Thread(target = do_something)
         t.daemon = True
         t.start() 
  
  ```
  If the thread finishes, proper clean up happens, and excel/python shuts down correctly.
  
  Is there any less ugly way to do use xw.Book() oder the app instance in the sub without creating zombies?
  
  Thanks for any help and/or suggestions!