Implement context manager protocol for Popen

src/nemu/subprocess_.py

@@ -212,6 +212,17 @@ class Popen(Subprocess):
             if getattr(self, k) is not None:
                 eintr_wrapper(os.close, v)
 
+    def __enter__(self):
+        return self
+
+    def __exit__(self, exc_type, exc_val, exc_tb):
+        if self.stdin is not None:
+            self.stdin.close()
+        if self.stdout is not None:
+            self.stdout.close()
+        if self.stderr is not None:
+            self.stderr.close()
+
     def communicate(self, input: bytes | str = None) -> tuple[bytes, bytes]:
         """See Popen.communicate."""
         # FIXME: almost verbatim from stdlib version, need to be removed or