Incompatibility with pypy interpreter

When trying to use jsonconversion with the pypy interpreter (e.g. pypy3.6 /custom/patch/python-jsonconversion/test/test_jsonobjects.py) the following error occurs:

```
File "/home/basti/develop/python-jsonconversion/test/test_jsonobjects.py", line 12, in
from testing_utils import convert_with_assertion
File "/home/basti/develop/python-jsonconversion/test/testing_utils.py", line 17, in
from jsonconversion.encoder import JSONObjectEncoder
File "/home/basti/develop/python-jsonconversion/src/jsonconversion/encoder.py", line 13, in
from json.encoder import JSONEncoder, _make_iterencode, encode_basestring, encode_basestring_ascii, INFINITY
ImportError: cannot import name '_make_iterencode'
```

The '_make_iterencode' is a protected member os json.encoder. Is there a possibility to only use the public API of json.encoder?