statsmodels is missing from optional dependencies
#### Code Sample, a copy-pastable example if possible

```
docs$ python make.py html

...

>>>-------------------------------------------------------------------------
Exception in /Users/tsmith/upstream/pandas/doc/source/basics.rst at block ending on line 762
Specify :okexcept: as an option in the ipython:: block to suppress this message
---------------------------------------------------------------------------
ModuleNotFoundError                       Traceback (most recent call last)
<ipython-input-138-5ad1e596ef83> in <module>()
----> 1 import statsmodels.formula.api as sm

ModuleNotFoundError: No module named 'statsmodels'
<<<-------------------------------------------------------------------------
```
#### Problem description

After setting up the development environment as described in the contributor guide, building the docs fails because statsmodels isn't included in the optional dependencies.

#### Expected Output

(none)

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

<details>

No module named 'dask'

INSTALLED VERSIONS
------------------
commit: 610a19a333c40ff67b8feb5f96c29cfae8495a3e
python: 3.6.4.final.0
python-bits: 64
OS: Darwin
OS-release: 17.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.0.dev0+307.g610a19a33.dirty
pytest: 3.6.3
pip: 10.0.1
setuptools: 40.0.0
Cython: 0.28.4
numpy: 1.14.5
scipy: 1.1.0
pyarrow: 0.9.0.post1
xarray: 0.10.7
IPython: 6.4.0
sphinx: 1.7.5
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.5
blosc: 1.5.1
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.5
feather: 0.4.0
matplotlib: 2.2.2
openpyxl: 2.5.4
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.5
lxml: 4.2.3
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.10
pymysql: 0.9.2
psycopg2: None
jinja2: 2.10
s3fs: 0.1.5
fastparquet: 0.1.5
pandas_gbq: None
pandas_datareader: None
gcsfs: 0.1.1

</details>

A possible complication is that statsmodels depends on pandas.

The imports I see are:

```
pandas$ grep -R "import statsmodels" .
./doc/source/whatsnew/v0.16.2.txt:   import statsmodels.formula.api as sm
./doc/source/basics.rst:   import statsmodels.formula.api as sm
```