BUG: maybe_convert_objects with convert_datetime
- [ Y] closes #19359
- [ Y] tests added / passed
- [ Y] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ Y] whatsnew entry: bug fix for GH19359

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19423?src=pr&el=h1) Report
> Merging [#19423](https://codecov.io/gh/pandas-dev/pandas/pull/19423?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/01882ba5b4c21b0caf2e6b9279fb01967aa5d650?src=pr&el=desc) will **decrease** coverage by `0.17%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19423/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/19423?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19423      +/-   ##
==========================================
- Coverage    91.8%   91.63%   -0.18%     
==========================================
  Files         152      150       -2     
  Lines       49215    48724     -491     
==========================================
- Hits        45181    44646     -535     
- Misses       4034     4078      +44
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90% <ø> (-0.19%)` | :arrow_down: |
| #single | `41.74% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19423?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/tseries/plotting.py](https://codecov.io/gh/pandas-dev/pandas/pull/19423/diff?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGxvdHRpbmcucHk=) | `0% <0%> (-100%)` | :arrow_down: |
| [pandas/plotting/\_compat.py](https://codecov.io/gh/pandas-dev/pandas/pull/19423/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb21wYXQucHk=) | `62% <0%> (-28.91%)` | :arrow_down: |
| [pandas/core/missing.py](https://codecov.io/gh/pandas-dev/pandas/pull/19423/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvbWlzc2luZy5weQ==) | `84.3% <0%> (-7.35%)` | :arrow_down: |
| [pandas/plotting/\_timeseries.py](https://codecov.io/gh/pandas-dev/pandas/pull/19423/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL190aW1lc2VyaWVzLnB5) | `60.82% <0%> (-4.49%)` | :arrow_down: |
| [pandas/core/reshape/tile.py](https://codecov.io/gh/pandas-dev/pandas/pull/19423/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS90aWxlLnB5) | `90.25% <0%> (-3.12%)` | :arrow_down: |
| [pandas/io/html.py](https://codecov.io/gh/pandas-dev/pandas/pull/19423/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2h0bWwucHk=) | `85.98% <0%> (-2.81%)` | :arrow_down: |
| [pandas/io/formats/format.py](https://codecov.io/gh/pandas-dev/pandas/pull/19423/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2Zvcm1hdHMvZm9ybWF0LnB5) | `96.24% <0%> (-2.01%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/19423/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.22% <0%> (-1.59%)` | :arrow_down: |
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/19423/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `95.52% <0%> (-0.84%)` | :arrow_down: |
| [pandas/util/\_decorators.py](https://codecov.io/gh/pandas-dev/pandas/pull/19423/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvX2RlY29yYXRvcnMucHk=) | `81.66% <0%> (-0.74%)` | :arrow_down: |
| ... and [69 more](https://codecov.io/gh/pandas-dev/pandas/pull/19423/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19423?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/19423?src=pr&el=footer). Last update [01882ba...081817e](https://codecov.io/gh/pandas-dev/pandas/pull/19423?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I've refactored the unit tests to use parametrize - learnt something new :)  can you rebase Rebased and updated proposed fix for review @lingster can you rebase and update closing as stale, if you want to continue working, pls ping and we can re-open. you will need to merge master. @jreback Is the only thing missing here the rebase and the cleanup from the previous code?
I can tidy this up so we can start retiring some old issues from the repo.
cc: @lingster  yes needs a rebase and updating according to comments