DEPR: remove statsmodels as a dependency
remove pd.ols, pd.fama_macbeth from top-level namespace

xref #11898
closes https://github.com/pandas-dev/pandas2/issues/26

previously deprecated in 0.18.0
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15353?src=pr&el=h1) Report
> Merging [#15353](https://codecov.io/gh/pandas-dev/pandas/pull/15353?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/d38d142aaac5ee046aabc32c1e6422f86c37dc41?src=pr&el=desc) will **increase** coverage by `2.38%`.


```diff
@@            Coverage Diff             @@
##           master   #15353      +/-   ##
==========================================
+ Coverage   86.32%   88.71%   +2.38%     
==========================================
  Files         141      133       -8     
  Lines       51163    49466    -1697     
==========================================
- Hits        44167    43884     -283     
+ Misses       6996     5582    -1414
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15353?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/stats/api.py](https://codecov.io/gh/pandas-dev/pandas/compare/d38d142aaac5ee046aabc32c1e6422f86c37dc41...6622ca05d3dc472ed2fdada888d7c2d8cef7171b?src=pr&el=tree#diff-cGFuZGFzL3N0YXRzL2FwaS5weQ==) | `100% <ø> (ø)` | :white_check_mark: |
| [pandas/util/print_versions.py](https://codecov.io/gh/pandas-dev/pandas/compare/d38d142aaac5ee046aabc32c1e6422f86c37dc41...6622ca05d3dc472ed2fdada888d7c2d8cef7171b?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvcHJpbnRfdmVyc2lvbnMucHk=) | `15.71% <ø> (ø)` | :white_check_mark: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/compare/d38d142aaac5ee046aabc32c1e6422f86c37dc41...6622ca05d3dc472ed2fdada888d7c2d8cef7171b?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `82.72% <ø> (+0.18%)` | :white_check_mark: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15353?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/15353?src=pr&el=footer). Last update [d38d142...9563740](https://codecov.io/gh/pandas-dev/pandas/compare/d38d142aaac5ee046aabc32c1e6422f86c37dc41...9563740551d537ca719e8e0554c7b9f2f2f472bb?src=pr&el=footer&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). > Merging #15353 into master will increase coverage by 2.38%

a nice way of increasing coverage :>