CLN: Cleanup top-level pandas namespace
Drops the following:

* `pd.pnow`
* `pd.match`
* `pd.groupby`
* `pd.get_store`

As well as:

* `pd.Expr`
* `pd.Term`

xref #15538.
xref #15940.

Hello @gfyoung! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/api.py`](https://github.com/gfyoung/pandas/blob/67c908d3c3a6ad1ad024887f0395373802a93c9b/pandas/core/api.py) !

 - There are no PEP8 issues in the file [`pandas/core/computation/api.py`](https://github.com/gfyoung/pandas/blob/67c908d3c3a6ad1ad024887f0395373802a93c9b/pandas/core/computation/api.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/period.py`](https://github.com/gfyoung/pandas/blob/67c908d3c3a6ad1ad024887f0395373802a93c9b/pandas/core/indexes/period.py) !

 - There are no PEP8 issues in the file [`pandas/io/api.py`](https://github.com/gfyoung/pandas/blob/67c908d3c3a6ad1ad024887f0395373802a93c9b/pandas/io/api.py) !

 - There are no PEP8 issues in the file [`pandas/io/pytables.py`](https://github.com/gfyoung/pandas/blob/67c908d3c3a6ad1ad024887f0395373802a93c9b/pandas/io/pytables.py) !

 - There are no PEP8 issues in the file [`pandas/tests/api/test_api.py`](https://github.com/gfyoung/pandas/blob/67c908d3c3a6ad1ad024887f0395373802a93c9b/pandas/tests/api/test_api.py) !

 - There are no PEP8 issues in the file [`pandas/tests/io/test_pytables.py`](https://github.com/gfyoung/pandas/blob/67c908d3c3a6ad1ad024887f0395373802a93c9b/pandas/tests/io/test_pytables.py) !

 - There are no PEP8 issues in the file [`pandas/tests/scalar/period/test_period.py`](https://github.com/gfyoung/pandas/blob/67c908d3c3a6ad1ad024887f0395373802a93c9b/pandas/tests/scalar/period/test_period.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23380?src=pr&el=h1) Report
> Merging [#23380](https://codecov.io/gh/pandas-dev/pandas/pull/23380?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f662c5f2a7b392ee96a43f055073c8523f8f0339?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23380/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23380?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23380      +/-   ##
==========================================
- Coverage   92.22%   92.22%   -0.01%     
==========================================
  Files         169      169              
  Lines       51261    51236      -25     
==========================================
- Hits        47277    47252      -25     
  Misses       3984     3984
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.66% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.23% <100%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23380?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/period.py](https://codecov.io/gh/pandas-dev/pandas/pull/23380/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9wZXJpb2QucHk=) | `92.22% <ø> (-0.06%)` | :arrow_down: |
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/23380/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `92.43% <ø> (-0.01%)` | :arrow_down: |
| [pandas/core/computation/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/23380/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tcHV0YXRpb24vYXBpLnB5) | `100% <ø> (ø)` | :arrow_up: |
| [pandas/io/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/23380/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2FwaS5weQ==) | `100% <100%> (ø)` | :arrow_up: |
| [pandas/core/api.py](https://codecov.io/gh/pandas-dev/pandas/pull/23380/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXBpLnB5) | `100% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23380?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/23380?src=pr&el=footer). Last update [f662c5f...67c908d](https://codecov.io/gh/pandas-dev/pandas/pull/23380?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @gfyoung 