Add statsmodels to optional dependencies
Some of the documentation uses methods from statsmodels, which isn't included in the optional dependency list.

Fixes #21911.

- [x] closes #xxxx
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

yep lgtm.
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21912?src=pr&el=h1) Report
> Merging [#21912](https://codecov.io/gh/pandas-dev/pandas/pull/21912?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2b51c968ca1e16a7fb517968576f8a9ab47ce1ed?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/21912/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/21912?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #21912   +/-   ##
=======================================
  Coverage   91.96%   91.96%           
=======================================
  Files         166      166           
  Lines       50323    50323           
=======================================
  Hits        46281    46281           
  Misses       4042     4042
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.36% <ø> (ø)` | :arrow_up: |
| #single | `42.23% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21912?src=pr&el=continue).
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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/21912?src=pr&el=footer). Last update [2b51c96...2126d51](https://codecov.io/gh/pandas-dev/pandas/pull/21912?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
