tools: fix union-attr type-check errors in OutputRedirect class

I'm very glad this is deprecated. The only reason I'm bothering to fix
this is because we can't remove it from the 8.0.x maintenance branch.

sopel/tools/__init__.py

@@ -160,7 +160,7 @@ def write(self, string):
 
         :param str string: the string to write
         """
-        if not self.quiet:
+        if not self.quiet and sys.__stderr__ and sys.__stdout__:
             try:
                 if self.stderr:
                     sys.__stderr__.write(string)
@@ -179,9 +179,9 @@ def write(self, string):
 
     def flush(self):
         """Flush the file writing buffer."""
-        if self.stderr:
+        if self.stderr and sys.__stderr__:
             sys.__stderr__.flush()
-        else:
+        elif sys.__stdout__:
             sys.__stdout__.flush()