Merge pull request RustPython#3915 from hy-kiera/textiobase_encoding_property

implement TextIOBase encoding attribute

Author: youknowone

vm/src/stdlib/io.rs

@@ -701,10 +701,16 @@ mod _io {
     // TextIO Base has no public constructor
     #[pyattr]
     #[pyclass(name = "_TextIOBase", base = "_IOBase")]
+    #[derive(Debug, PyPayload)]
     struct _TextIOBase;
 
     #[pyimpl(flags(BASETYPE))]
-    impl _TextIOBase {}
+    impl _TextIOBase {
+        #[pyproperty]
+        fn encoding(&self, vm: &VirtualMachine) -> PyObjectRef {
+            vm.ctx.none()
+        }
+    }
 
     #[derive(FromArgs, Clone)]
     struct BufferSize {