renderer.py checks platform for windows, rather than checking if the Output object is Windows output

While working with contrib/ssh on my Windows10 box, I found a problem with renderer.py

Before calling get_rows_below_cursor_position() in request_absolute_cursor_position() we check is_windows(). This seems incorrect to me, because the platform is not the issue, but the Output is. When creating an SSHD server on Windows, the output is actually a Vt100, but this platform check bogs that up. There are a few other calls to is_windows() in renderer.py but I didn't investigate them.

This is how I'm handling it locally:

--- a/prompt_toolkit/renderer.py        2020-05-29 13:15:17.928292000 -0700
+++ b/prompt_toolkit/renderer.py        2020-05-29 13:21:18.235659400 -0700
@@ -422,7 +422,7 @@

         # For Win32, we have an API call to get the number of rows below the
         # cursor.
-        elif is_windows():
+        elif type(self.output).__name__ in ('Win32Output','Windows10_Output'):
             self._min_available_height = self.output.get_rows_below_cursor_position()

         # Use CPR.

Start in prompt_toolkit/renderer.py, especially request_absolute_cursor_position() and the call to get_rows_below_cursor_position(). Inspect how the Output object is represented for Win32, Windows10, and Vt100 outputs before reviewing the other is_windows() checks. Done means platform-independent output handling works for the Windows SSHD/Vt100 case without breaking native Windows output.