build: add "dumb" platform check for mypy

Mypy doesn't understand some clever version checks. This adds an
otherwise un-needed assert to force mypy to ignore the file on linux

Build-exe-and-msi.py

@@ -28,6 +28,9 @@ if sys.platform == 'win32':
 
 else:
     raise AssertionError(f'Unsupported platform {sys.platform}')
+
+# This added to make mypy happy
+assert sys.platform == 'win32'
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