statsmodels is missing from optional dependencies

[tdsmith]

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()

No module named 'dask'

INSTALLED VERSIONS

commit: 610a19a
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

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