str vs bytes confusion in ujson on Python 3
#### Code Sample, a copy-pastable example if possible

```python
import numpy as np
import pandas._libs.json as ujson

ujson.encode(np.array(1))
```
#### Problem description

On CPython 3.5, this raises TypeError, as expected, but the output contains garbage bytes:
```
Traceback (most recent call last):
  File "bug.py", line 4, in <module>
    ujson.encode(np.array(1))
TypeError: � (0d array) is not JSON serializable at the moment
```
On pypy3, it segfaults.

The problem comes from this code: https://github.com/pandas-dev/pandas/blob/775099cde4a2424dfb369d79b8b6e2687e5af7f9/pandas/_libs/src/ujson/python/objToJSON.c#L1925-L1929 It calls `PyString_AS_STRING` (=`PyBytes_AS_STRING` on py3k) on the PyUnicodeObject returned by `PyObject_Repr`.

I haven't looked very carefully, but there seem to be a few more similar misuses of `PyString_AS_STRING` in the same file.

#### Expected Output

```
Traceback (most recent call last):
  File "bug.py", line 4, in <module>
    ujson.encode(np.array(1))
TypeError: array(1) (0d array) is not JSON serializable at the moment
```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-135-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: fr_FR.UTF-8

pandas: 0.21.1
pytest: 3.2.5
pip: 9.0.1
setuptools: 37.0.0
Cython: None
numpy: 1.13.3
scipy: None
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None


</details>
