Implement seekable, readable, and writable methods

https://docs.python.org/2/library/io.html#io.IOBase.seekable
https://docs.python.org/3/library/io.html#io.IOBase.seekable
https://docs.python.org/2/library/io.html#io.IOBase.writable
https://docs.python.org/3/library/io.html#io.IOBase.writable
https://docs.python.org/2/library/io.html#io.IOBase.readable
https://docs.python.org/3/library/io.html#io.IOBase.readable

These methods should be implemented on all file like objects.

boltons/ioutils.py

@@ -170,6 +170,12 @@ class SpooledIOBase(object):
     def seekable(self):
         return True
 
+    def readable(self):
+        return True
+
+    def writable(self):
+        return True
+
     __next__ = next
 
     def __len__(self):