DEPR: favour sep over delimiter in pd.read_csv
- [x] closes #21996
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

following discussion in #21996 and #7662, pandas uses `sep` extensively and in `to_csv` counterpart too. This PR deprecates `delimiter` argument from 0.24.0

comments welcome.
Hello @minggli! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/io/parsers.py`](https://github.com/minggli/pandas/blob/46014115c5267c560cfbef8b1f635d8fa5e21f85/pandas/io/parsers.py) !

 - There are no PEP8 issues in the file [`pandas/tests/io/parser/common.py`](https://github.com/minggli/pandas/blob/46014115c5267c560cfbef8b1f635d8fa5e21f85/pandas/tests/io/parser/common.py) !

 - There are no PEP8 issues in the file [`pandas/tests/io/parser/dialect.py`](https://github.com/minggli/pandas/blob/46014115c5267c560cfbef8b1f635d8fa5e21f85/pandas/tests/io/parser/dialect.py) !

 As mentioned in the issue, I think we should decide on deprecating this or not together with the discussion in https://github.com/pandas-dev/pandas/issues/22639 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23158?src=pr&el=h1) Report
> Merging [#23158](https://codecov.io/gh/pandas-dev/pandas/pull/23158?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f9d237b215acded23c5efc99f23f38adea818e91?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23158/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23158?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23158      +/-   ##
==========================================
+ Coverage   92.14%   92.14%   +<.01%     
==========================================
  Files         170      170              
  Lines       50992    50993       +1     
==========================================
+ Hits        46985    46986       +1     
  Misses       4007     4007
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.56% <100%> (ø)` | :arrow_up: |
| #single | `42.32% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23158?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/23158/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.6% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23158?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/23158?src=pr&el=footer). Last update [f9d237b...f2f1362](https://codecov.io/gh/pandas-dev/pandas/pull/23158?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/23158?src=pr&el=h1) Report
> Merging [#23158](https://codecov.io/gh/pandas-dev/pandas/pull/23158?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/92fd46c8bf719746b4c720bf7599d4080b319c8d?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23158/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23158?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23158      +/-   ##
==========================================
+ Coverage   92.22%   92.22%   +<.01%     
==========================================
  Files         169      169              
  Lines       50911    50912       +1     
==========================================
+ Hits        46954    46955       +1     
  Misses       3957     3957
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.65% <100%> (ø)` | :arrow_up: |
| #single | `42.29% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23158?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/parsers.py](https://codecov.io/gh/pandas-dev/pandas/pull/23158/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3BhcnNlcnMucHk=) | `95.6% <100%> (ø)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23158?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/23158?src=pr&el=footer). Last update [92fd46c...2d7b7ab](https://codecov.io/gh/pandas-dev/pandas/pull/23158?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @WillAyd actioned. but something unrelated seems to break azure CI @jreback, cool, just ping if anything is decided or needs change! thanks 👍  will do @minggli  Closing as stale. If we want another PR here should continue discussion back in original issue