df.query() does not support column name 'class'
#### Code Sample, a copy-pastable example if possible

```python
indices_to_plot = df.query('class>0')
```
#### Problem description
Above code results in this error traceback:
```python
Traceback (most recent call last):

  File "/Users/klay6683/miniconda3/envs/stable/lib/python3.6/site-packages/IPython/core/interactiveshell.py", line 2910, in run_code
    exec(code_obj, self.user_global_ns, self.user_ns)

  File "<ipython-input-33-6e077c50ac68>", line 2, in <module>
    indices_to_plot = df.query('class>0')

  File "/Users/klay6683/miniconda3/envs/stable/lib/python3.6/site-packages/pandas/core/frame.py", line 2297, in query
    res = self.eval(expr, **kwargs)

  File "/Users/klay6683/miniconda3/envs/stable/lib/python3.6/site-packages/pandas/core/frame.py", line 2366, in eval
    return _eval(expr, inplace=inplace, **kwargs)

  File "/Users/klay6683/miniconda3/envs/stable/lib/python3.6/site-packages/pandas/core/computation/eval.py", line 290, in eval
    truediv=truediv)

  File "/Users/klay6683/miniconda3/envs/stable/lib/python3.6/site-packages/pandas/core/computation/expr.py", line 732, in __init__
    self.terms = self.parse()

  File "/Users/klay6683/miniconda3/envs/stable/lib/python3.6/site-packages/pandas/core/computation/expr.py", line 749, in parse
    return self._visitor.visit(self.expr)

  File "/Users/klay6683/miniconda3/envs/stable/lib/python3.6/site-packages/pandas/core/computation/expr.py", line 310, in visit
    node = ast.fix_missing_locations(ast.parse(clean))

  File "/Users/klay6683/miniconda3/envs/stable/lib/python3.6/ast.py", line 35, in parse
    return compile(source, filename, mode, PyCF_ONLY_AST)

  File "<unknown>", line 1
    class >0
          ^
SyntaxError: invalid syntax
```

My column names are "occ_id, class, et, radius, lon, width, type" and if I execute this query on another column, it works fine:

```python
indices_to_plot = df.query('et>0')
```
Only the column named 'class' seems to fail.

#### Expected Output
Sub selection of the dataframe according to the query.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Darwin
OS-release: 16.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.21.0
pytest: 3.2.3
pip: 9.0.1
setuptools: 36.6.0
Cython: 0.27.3
numpy: 1.13.3
scipy: 0.19.1
pyarrow: None
xarray: 0.9.6
IPython: 6.2.1
sphinx: 1.6.5
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.0
openpyxl: None
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: None
lxml: None
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
