Explicit windows check before sigkill for mypy

src/openflexure_microscope_server/stitching.py

@@ -11,6 +11,7 @@ import os
 import shlex
 import signal
 import subprocess
+import sys
 import threading
 from typing import IO, Optional
 
@@ -221,7 +222,8 @@ class PreviewStitcher(BaseStitcher):
             except lt.exceptions.InvocationCancelledError as e:
                 with self._popen_lock:
                     if self._popen_obj is not None:
-                        if IS_WINDOWS:
+                        # mypy requires the check to be here
+                        if sys.platform == "win32":
                             # Windows has no SIGKILL
                             self._popen_obj.kill()
                         else: