CLN: replace _interleave_dtype with _find_common_type
xref #15736 
xref #12780 
cc @sdementen 

you can rebase on top of this; should make that PR a lot simpler. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15765?src=pr&el=h1) Report
> Merging [#15765](https://codecov.io/gh/pandas-dev/pandas/pull/15765?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/783ae69f0edc350bdf7b20932351c79a5a1fad3c?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `100%`.


```diff
@@            Coverage Diff             @@
##           master   #15765      +/-   ##
==========================================
- Coverage   91.01%   90.99%   -0.03%     
==========================================
  Files         143      143              
  Lines       49395    49371      -24     
==========================================
- Hits        44959    44927      -32     
- Misses       4436     4444       +8
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15765?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/types/cast.py](https://codecov.io/gh/pandas-dev/pandas/compare/783ae69f0edc350bdf7b20932351c79a5a1fad3c...d47264683bf51d5a97e4affcaf835d714937cc43?src=pr&el=tree#diff-cGFuZGFzL3R5cGVzL2Nhc3QucHk=) | `85.31% <100%> (+0.2%)` | :arrow_up: |
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/compare/783ae69f0edc350bdf7b20932351c79a5a1fad3c...d47264683bf51d5a97e4affcaf835d714937cc43?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `93.62% <100%> (ø)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/compare/783ae69f0edc350bdf7b20932351c79a5a1fad3c...d47264683bf51d5a97e4affcaf835d714937cc43?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/783ae69f0edc350bdf7b20932351c79a5a1fad3c...d47264683bf51d5a97e4affcaf835d714937cc43?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.86% <0%> (-0.1%)` | :arrow_down: |
| [pandas/tseries/index.py](https://codecov.io/gh/pandas-dev/pandas/compare/783ae69f0edc350bdf7b20932351c79a5a1fad3c...d47264683bf51d5a97e4affcaf835d714937cc43?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvaW5kZXgucHk=) | `95.29% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15765?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/15765?src=pr&el=footer). Last update [783ae69...d472646](https://codecov.io/gh/pandas-dev/pandas/compare/783ae69f0edc350bdf7b20932351c79a5a1fad3c...d47264683bf51d5a97e4affcaf835d714937cc43?src=pr&el=footer&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).