Missing encoding argument for pipe open

[aklein3]

The call to open() to terminate MiniZinc on Windows omits the encoding argument. This triggers an EncodingWarning if warnings are enabled (-W default).

The fix is as follows:

--- a/src/minizinc/instance.py
+++ b/src/minizinc/instance.py
@@ -675,7 +675,8 @@ class Instance(Model):
                 # First, terminate the process
                 if sys.platform == "win32":
                     with open(
-                        f"\\\\.\\pipe\\minizinc-{proc.pid}", mode="w"
+                        f"\\\\.\\pipe\\minizinc-{proc.pid}", mode="w",
+                        encoding="utf8"
                     ) as named_pipe:
                         # Trigger MiniZinc termination
                         named_pipe.write("")