Delegate unknown attrs to target in WrappingIO

Same idea as fbb2f4d but more generic.
Only the flushing should be wrapped, everything else should behave like
it would on the target.

The issue arises while trying to access attributes like `encoding` or
`fileno`.

Author: william-andre

progressbar/utils.py

@@ -199,8 +199,8 @@ def __init__(self, target: types.IO, capturing: bool = False,
         self.listeners = listeners or set()
         self.needs_clear = False
 
-    def isatty(self):  # pragma: no cover
-        return self.target.isatty()
+    def __getattr__(self, name):  # pragma: no cover
+        return getattr(self.target, name)
 
     def write(self, value: str) -> None:
         if self.capturing: