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?

Start by running test/test_jsonobjects.py with PyPy and inspect the failing import in src/jsonconversion/encoder.py, with test/testing_utils.py showing how the encoder is used. The work is done when the tests run under PyPy without relying on the unavailable json.encoder member.

Written by the indexing model from the issue text.