DOC: Added statsmodels to show_versions
xref https://github.com/statsmodels/statsmodels/issues/3580
I have nothing against adding statsmodels per se, but the output is getting quite long, so we should maybe restrict it to actual dependencies? AFAIK we don't use statsmodel anywhere in our code? (apart from some doc examples) 
Or is it useful when working on statsmodels/pandas compat? # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=h1) Report
> Merging [#15921](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/763197c3422d46b8e4cc807d58a63c6be6a9a288?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15921/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15921      +/-   ##
==========================================
- Coverage   90.96%   90.95%   -0.01%     
==========================================
  Files         145      145              
  Lines       49534    49534              
==========================================
- Hits        45057    45056       -1     
- Misses       4477     4478       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.72% <ø> (-0.01%)` | :arrow_down: |
| #single | `40.63% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/print\_versions.py](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvcHJpbnRfdmVyc2lvbnMucHk=) | `15.71% <ø> (ø)` | :arrow_up: |
| [pandas/tseries/index.py](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvaW5kZXgucHk=) | `95.33% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=footer). Last update [763197c...2336be9](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=h1) Report
> Merging [#15921](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/763197c3422d46b8e4cc807d58a63c6be6a9a288?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/15921/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #15921      +/-   ##
==========================================
- Coverage   90.96%   90.95%   -0.01%     
==========================================
  Files         145      145              
  Lines       49534    49534              
==========================================
- Hits        45057    45056       -1     
- Misses       4477     4478       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.72% <ø> (-0.01%)` | :arrow_down: |
| #single | `40.63% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/util/print\_versions.py](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvcHJpbnRfdmVyc2lvbnMucHk=) | `15.71% <ø> (ø)` | :arrow_up: |
| [pandas/tseries/index.py](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvaW5kZXgucHk=) | `95.33% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=footer). Last update [763197c...2336be9](https://codecov.io/gh/pandas-dev/pandas/pull/15921?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). I agree with @jorisvandenbossche here. its not a dependence (any more); we took it out https://github.com/pandas-dev/pandas/pull/15353 Works for me. 