Fix Py3 build when Cython is not available

zstd/_bindings.pyx

+# cython: c_string_encoding=utf8
+
 from cpython cimport (PyObject, PY_LONG_LONG,
     PyBytes_FromStringAndSize as _PyBytes_FromStringAndSize,
     PyBytes_AS_STRING as _PyBytes_AS_STRING)
@@ -37,11 +39,7 @@ def getFrameContentSize(bytes string not None):
     return _getFrameContentSize(_PyBytes_AS_STRING(string), len(string))
 
 def getErrorName(code):
-    # why can't I just cast to str ?
-    IF str is bytes:
-        return ZSTD_getErrorName(code)
-    ELSE:
-        return ZSTD_getErrorName(code).decode()
+    return <str>ZSTD_getErrorName(code)
 
 global error