prevented string dtypes from being split into chars in pytest_helpers.

Author: djl11

array_api_tests/pytest_helpers.py

@@ -121,7 +121,7 @@ def assert_dtype(
         >>> assert_dtype('sum', x, out.dtype, default_int)
 
     """
-    in_dtypes = in_dtype if isinstance(in_dtype, Sequence) else [in_dtype]
+    in_dtypes = in_dtype if isinstance(in_dtype, Sequence) and not isinstance(in_dtype, str) else [in_dtype]
     f_in_dtypes = dh.fmt_types(tuple(in_dtypes))
     f_out_dtype = dh.dtype_to_name[out_dtype]
     if expected is None: