outp: fixed double/string quotes

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 f'{self.header}\n{"\n".join(map(str, self.blocks))}{self.footer}'
 
     def to_dataframe(self):
         """