ENH union_categoricals supports ignore_order GH13410
xref #13410 (ignore_order portion)

Thanks for the comments.  Pull request has been updated. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15219?src=pr&el=h1) Report
> Merging [#15219](https://codecov.io/gh/pandas-dev/pandas/pull/15219?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/be4a63fe791e27c2f8a9ae4f3a419ccc255c1b5b?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.


```diff
@@            Coverage Diff             @@
##           master   #15219      +/-   ##
==========================================
+ Coverage   90.37%   90.37%   +<.01%     
==========================================
  Files         135      135              
  Lines       49464    49466       +2     
==========================================
+ Hits        44702    44705       +3     
+ Misses       4762     4761       -1
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/15219?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/types/concat.py](https://codecov.io/gh/pandas-dev/pandas/compare/be4a63fe791e27c2f8a9ae4f3a419ccc255c1b5b...d278d623b7ed00317a870b3649c5692c284d2864?src=pr&el=tree#diff-cGFuZGFzL3R5cGVzL2NvbmNhdC5weQ==) | `98.06% <100%> (+0.01%)` | :white_check_mark: |
| [pandas/core/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/be4a63fe791e27c2f8a9ae4f3a419ccc255c1b5b...d278d623b7ed00317a870b3649c5692c284d2864?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY29tbW9uLnB5) | `91.36% <ø> (+0.33%)` | :white_check_mark: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/15219?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/15219?src=pr&el=footer). Last update [be4a63f...e9d00de](https://codecov.io/gh/pandas-dev/pandas/compare/be4a63fe791e27c2f8a9ae4f3a419ccc255c1b5b...e9d00dec17bd6952048a46ae130ab8e79cba286b?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments). @jorisvandenbossche Any other actions on my part? @js3711 minor corrections and a flake issue you can see

``git diff master | flake8 --diff`` to see locally

ping when pushed / green. can you rebase / update  I'll update this weekend.  Apologies for the delay. @jreback  thanks for the feedback.  I made the suggested changes and submitted a new PR. thanks for the PR @js3711 