BLD: Bump Cython version from 0.23 to 0.24
- [X] closes #18613 
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [X] whatsnew entry

For the whatsnew:

- Used the same whatsnew message as was used during the last cython version bump in 0.20
- Moved some previous entries that were under the main "Backwards incompatible API changes" section to the "Other API Changes" subsection.
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18623?src=pr&el=h1) Report
> Merging [#18623](https://codecov.io/gh/pandas-dev/pandas/pull/18623?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6e56195fcb021e7c53301722804087ef2a83bc39?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18623/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18623?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18623      +/-   ##
==========================================
- Coverage   91.46%   91.45%   -0.02%     
==========================================
  Files         157      157              
  Lines       51449    51449              
==========================================
- Hits        47060    47051       -9     
- Misses       4389     4398       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.32% <ø> (ø)` | :arrow_up: |
| #single | `40.6% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18623?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18623/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18623/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18623?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/18623?src=pr&el=footer). Last update [6e56195...97cc75f](https://codecov.io/gh/pandas-dev/pandas/pull/18623?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/18623?src=pr&el=h1) Report
> Merging [#18623](https://codecov.io/gh/pandas-dev/pandas/pull/18623?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/6e56195fcb021e7c53301722804087ef2a83bc39?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18623/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/18623?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18623      +/-   ##
==========================================
- Coverage   91.46%   91.45%   -0.02%     
==========================================
  Files         157      157              
  Lines       51449    51449              
==========================================
- Hits        47060    47051       -9     
- Misses       4389     4398       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.32% <ø> (ø)` | :arrow_up: |
| #single | `40.6% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18623?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18623/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18623/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18623?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/18623?src=pr&el=footer). Last update [6e56195...97cc75f](https://codecov.io/gh/pandas-dev/pandas/pull/18623?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/18623?src=pr&el=h1) Report
> Merging [#18623](https://codecov.io/gh/pandas-dev/pandas/pull/18623?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2c903d594299b2441d4742e777a10e8c76557386?src=pr&el=desc) will **decrease** coverage by `0.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18623/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18623?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18623      +/-   ##
==========================================
- Coverage   91.59%   91.58%   -0.02%     
==========================================
  Files         155      155              
  Lines       51255    51255              
==========================================
- Hits        46949    46941       -8     
- Misses       4306     4314       +8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.44% <ø> (ø)` | :arrow_up: |
| #single | `40.67% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18623?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18623/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18623/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/18623/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.68% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18623?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/18623?src=pr&el=footer). Last update [2c903d5...d7fd50c](https://codecov.io/gh/pandas-dev/pandas/pull/18623?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 lgtm. ping on green. ping @jreback : green @jschendel thanks!