Use backend's parse_basecoro

This one has flown under the radar for a long time, but while inspecting
some pytuple checks in the C extension it became evident we were always
using the generic version instead.

Signed-off-by: Rodrigo Tobar <[email protected]>

src/ijson/common.py

@@ -386,7 +386,7 @@ def parse(source, buf_size=64*1024, **config):
             )
         elif is_iterable(source):
             return utils.coros2gen(source,
-                (parse_basecoro, (), {})
+                (backend['parse_basecoro'], (), {})
             )
         raise ValueError("Unknown source type: %r" % type(source))
     return parse