AttributeError: module 'pandas' has no attribute 'plotting' with tox
`pandas` 0.20.3 fails to import on every test when running tox.

- `tox` packages installed, besides my package:

`cycler==0.10.0, matplotlib==2.0.2, numexpr==2.6.2, numpy==1.13.1, pandas==0.20.3, py==1.4.34, pyparsing==2.2.0, pytest==3.2.2, pytest-travis-fold==1.2.0, python-dateutil==2.6.1, pytz==2017.2, scipy==0.19.1, six==1.10.0, tables==3.4.2`

- `platform linux -- Python 3.5.2, pytest-3.2.2, py-1.4.34, pluggy-0.4.0`

- all tests fail with the following error:

```
myfile.py:16: in <module>
    import pandas as pd
.tox/3.5-nocov/lib/python3.5/site-packages/pandas/__init__.py:51: in <module>
    plot_params = pandas.plotting._style._Options(deprecated=True)
E   AttributeError: module 'pandas' has no attribute 'plotting'
```

Testing the same import in a `virtualenv` without `tox` works correctly.

Looks like it is a problem with `pandas`, but could also be something regarding `tox` (or even `pytest`). Hopefully, someone more experienced with `tox` can guide me here.

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

Taken from the `virtualenv` I created to test outside `tox`.

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.4.0-93-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: pt_PT.UTF-8

pandas: 0.20.3
pytest: 3.2.2
pip: 9.0.1
setuptools: 28.8.0
Cython: None
numpy: 1.13.1
scipy: 0.19.1
xarray: None
IPython: None
sphinx: 1.6.3
patsy: None
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: 3.4.2
numexpr: 2.6.2
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: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: None
None

</details>
