CLN: Cleanup toplevel namespace shims
Removes the following:

* `pandas.types`
* `pandas.computation`
* `pandas.util.decorators`

xref #16157.
xref #16250.
Hello @gfyoung! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/tests/api/test_api.py`](https://github.com/gfyoung/pandas/blob/8bacfdd808429a69549dcd3b0a1e555b3153bc74/pandas/tests/api/test_api.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23386?src=pr&el=h1) Report
> Merging [#23386](https://codecov.io/gh/pandas-dev/pandas/pull/23386?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/62a15fa4071d0711b2b97771b981d527c1c4af10?src=pr&el=desc) will **increase** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23386/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23386?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23386      +/-   ##
==========================================
+ Coverage   92.16%   92.17%   +0.01%     
==========================================
  Files         166      163       -3     
  Lines       51224    51213      -11     
==========================================
- Hits        47212    47207       -5     
+ Misses       4012     4006       -6
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.61% <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/23386?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/23386?src=pr&el=footer). Last update [62a15fa...9824fe6](https://codecov.io/gh/pandas-dev/pandas/pull/23386?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 can you rebase @jreback : Rebased and green. PTAL.

 thanks @gfyoung 