Not possible to assign string values to columns using DataFrame.eval() under Python 3.6
#### Code Sample, a copy-pastable example if possible

```python
>>> import pandas as pd
>>> df = pd.DataFrame(data=[(1,2),(3,4)], columns=['a', 'b'])
>>> df
   a  b
0  1  2
1  3  4
>>> df.eval("c='foo'")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/lib/python3.6/site-packages/pandas-0.19.2-py3.6-linux-x86_64.egg/pandas/core/frame.py", line 2279, in eval
    return _eval(expr, inplace=inplace, **kwargs)
  File "/lib/python3.6/site-packages/pandas-0.19.2-py3.6-linux-x86_64.egg/pandas/computation/eval.py", line 266, in eval
    ret = eng_inst.evaluate()
  File "/lib/python3.6/site-packages/pandas-0.19.2-py3.6-linux-x86_64.egg/pandas/computation/engines.py", line 76, in evaluate
    res = self._evaluate()
  File "/lib/python3.6/site-packages/pandas-0.19.2-py3.6-linux-x86_64.egg/pandas/computation/engines.py", line 123, in _evaluate
    return ne.evaluate(s, local_dict=scope, truediv=truediv)
  File "/lib/python3.6/site-packages/numexpr/necompiler.py", line 789, in evaluate
    zip(names, arguments)]
  File "/lib/python3.6/site-packages/numexpr/necompiler.py", line 788, in <listcomp>
    signature = [(name, getType(arg)) for (name, arg) in
  File "/lib/python3.6/site-packages/numexpr/necompiler.py", line 686, in getType
    raise ValueError("unknown type %s" % a.dtype.name)
ValueError: unknown type str96

# Switching type to bytes produces another error
>>> df.eval(b"c='foo'")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/lib/python3.6/site-packages/pandas-0.19.2-py3.6-linux-x86_64.egg/pandas/core/frame.py", line 2279, in eval
    return _eval(expr, inplace=inplace, **kwargs)
  File "/lib/python3.6/site-packages/pandas-0.19.2-py3.6-linux-x86_64.egg/pandas/computation/eval.py", line 261, in eval
    truediv=truediv)
  File "/lib/python3.6/site-packages/pandas-0.19.2-py3.6-linux-x86_64.egg/pandas/computation/expr.py", line 725, in __init__
    self.terms = self.parse()
  File "/lib/python3.6/site-packages/pandas-0.19.2-py3.6-linux-x86_64.egg/pandas/computation/expr.py", line 742, in parse
    return self._visitor.visit(self.expr)
  File "/lib/python3.6/site-packages/pandas-0.19.2-py3.6-linux-x86_64.egg/pandas/computation/expr.py", line 312, in visit
    return visitor(node, **kwargs)
  File "/lib/python3.6/site-packages/pandas-0.19.2-py3.6-linux-x86_64.egg/pandas/computation/expr.py", line 318, in visit_Module
    return self.visit(expr, **kwargs)
  File "/lib/python3.6/site-packages/pandas-0.19.2-py3.6-linux-x86_64.egg/pandas/computation/expr.py", line 312, in visit
    return visitor(node, **kwargs)
  File "/lib/python3.6/site-packages/pandas-0.19.2-py3.6-linux-x86_64.egg/pandas/computation/expr.py", line 321, in visit_Expr
    return self.visit(node.value, **kwargs)
  File "/lib/python3.6/site-packages/pandas-0.19.2-py3.6-linux-x86_64.egg/pandas/computation/expr.py", line 311, in visit
    visitor = getattr(self, method)
AttributeError: 'PandasExprVisitor' object has no attribute 'visit_Bytes'
>>>
```
#### Problem description
It does not seem possible to assign string values to columns under Python 3.6 (probably under all Python 3.X versions but I've only tried it with 3.6).

On python 2.7.12 the above works fine.

#### Expected Output
A data frame with all rows in column c set to the string 'foo'.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.0.final.0
python-bits: 64
OS: Linux
OS-release: 3.13.0-83-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.2
nose: None
pip: 9.0.1
setuptools: 34.1.0
Cython: None
numpy: 1.12.0
scipy: None
statsmodels: None
xarray: None
IPython: None
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: None
numexpr: 2.6.2
matplotlib: None
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
boto: None
pandas_datareader: None

</details>
