outp: fixed backslashes in f-string

Author: BitterB0NG0

src/pymcnp/Outp.py

@@ -109,7 +109,7 @@ def to_mcnp(self):
             OUTP for ``Outp``.
         """
 
-        return f'{self.header}\n{"\n".join(map(str, self.blocks))}{self.footer}'
+        return self.header + '\n' + '\n'.join(map(str, self.blocks)) + self.footer
 
     def to_dataframe(self):
         """