DataFrame.eval errors with AttributeError: 'UnaryOp'
#### Code Sample, a copy-pastable example if possible

```
def test_unary():
    df = pd.DataFrame({'x': np.array([0.11, 0], dtype=np.float32)})
    res = df.eval('(x > 0.1) | (x < -0.1)')
    assert np.array_equal(res, np.array([True, False])), res
```

#### Problem description

This is related to #11235. 
on python 3.6, pandas 20.1, this raises an error the traceback ends with:

```
  File ".../envs/py3/lib/python3.6/site-packages/pandas/core/computation/expr.py", line 370, in _maybe_downcast_constants
    name = self.env.add_tmp(np.float32(right.value))
AttributeError: 'UnaryOp' object has no attribute 'value'
```
In that case the right is -(0.1)


<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Linux
OS-release: 4.8.0-49-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.20.1
pytest: None
pip: 9.0.1
setuptools: 27.2.0
Cython: 0.25.2
numpy: 1.12.1
scipy: 0.19.0
xarray: None
IPython: 6.0.0
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.0.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: None
s3fs: None
pandas_gbq: None
pandas_datareader: None
</details>

Another example:

```python
>>> df = pd.DataFrame({'x':[1,2,3,4,5]})
>>> df.eval('x.shift(-1)')
```