py: Remove IOError since it's deprecated; use OSError instead.

In CPython IOError (and EnvironmentError) is deprecated and aliased to
OSError.  All modules that used to raise IOError now raise OSError (or a
derived exception).

In Micro Python we never used IOError (except 1 place, incorrectly) and
so don't need to keep it.

See http://legacy.python.org/dev/peps/pep-3151/ for background.
diff --git a/py/objexcept.c b/py/objexcept.c
index 7f07365..9b22acd 100644
--- a/py/objexcept.c
+++ b/py/objexcept.c
@@ -224,10 +224,10 @@
   MP_DEFINE_EXCEPTION(AssertionError, Exception)
   MP_DEFINE_EXCEPTION(AttributeError, Exception)
   //MP_DEFINE_EXCEPTION(BufferError, Exception)
-  //MP_DEFINE_EXCEPTION(EnvironmentError, Exception)
+  //MP_DEFINE_EXCEPTION(EnvironmentError, Exception) use OSError instead
   MP_DEFINE_EXCEPTION(EOFError, Exception)
   MP_DEFINE_EXCEPTION(ImportError, Exception)
-  MP_DEFINE_EXCEPTION(IOError, Exception)
+  //MP_DEFINE_EXCEPTION(IOError, Exception) use OSError instead
   MP_DEFINE_EXCEPTION(LookupError, Exception)
     MP_DEFINE_EXCEPTION_BASE(LookupError)
     MP_DEFINE_EXCEPTION(IndexError, LookupError)