outp: fixed f-strings with backslashes 2

Author: BitterB0NG0

src/pymcnp/outp/Mcnp.py

@@ -137,10 +137,13 @@ def to_mcnp(self):
             OUTP for ``Mcnp``.
         """
 
+        lines = '\n'.join(self.lines)
+        messages = '\n'.join(self.messages)
+
         return f"""
 1mcnp     {self.version}{self.code_date}{self.run_date}{self.run_time}
  *************************************************************************                 {self.probid}
  {self.command}
-{'\n'.join(self.lines)}
-{'\n'.join(self.messages)}
+{lines}
+{messages}
 """[1:-1]