tests/io: Add simple IOBase test.

tests/io/iobase.py

@@ -0,0 +1,19 @@
+try:
+    import uio as io
+except:
+    import io
+
+try:
+    io.IOBase
+except AttributeError:
+    print('SKIP')
+    raise SystemExit
+
+
+class MyIO(io.IOBase):
+    def write(self, buf):
+        # CPython and uPy pass in different types for buf (str vs bytearray)
+        print('write', len(buf))
+        return len(buf)
+
+print('test', file=MyIO())