Fix (Series|DataFrame).interpolate for datetime dtypes
- [x] closes #19199
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19291?src=pr&el=h1) Report
> Merging [#19291](https://codecov.io/gh/pandas-dev/pandas/pull/19291?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/f0cd23ca6485940e3a6572cc4f5afdce4c3921ab?src=pr&el=desc) will **increase** coverage by `0.03%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/19291/graphs/tree.svg?height=150&width=650&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/19291?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #19291      +/-   ##
==========================================
+ Coverage   91.53%   91.57%   +0.03%     
==========================================
  Files         150      148       -2     
  Lines       48738    48913     +175     
==========================================
+ Hits        44611    44790     +179     
+ Misses       4127     4123       -4
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.94% <100%> (+0.04%)` | :arrow_up: |
| #single | `41.66% <5.88%> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/19291?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/pull/19291/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `94.54% <100%> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/19291/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `95.91% <100%> (ø)` | :arrow_up: |
| [pandas/core/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/19291/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvY2F0ZWdvcmljYWwucHk=) | `95.78% <0%> (-4.22%)` | :arrow_down: |
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/19291/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `92.17% <0%> (-0.94%)` | :arrow_down: |
| [pandas/io/pytables.py](https://codecov.io/gh/pandas-dev/pandas/pull/19291/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL3B5dGFibGVzLnB5) | `92.45% <0%> (ø)` | :arrow_up: |
| [pandas/core/reshape/reshape.py](https://codecov.io/gh/pandas-dev/pandas/pull/19291/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcmVzaGFwZS9yZXNoYXBlLnB5) | `100% <0%> (ø)` | :arrow_up: |
| [pandas/compat/pickle\_compat.py](https://codecov.io/gh/pandas-dev/pandas/pull/19291/diff?src=pr&el=tree#diff-cGFuZGFzL2NvbXBhdC9waWNrbGVfY29tcGF0LnB5) | `75.6% <0%> (ø)` | :arrow_up: |
| [pandas/core/arrays/\_\_init\_\_.py](https://codecov.io/gh/pandas-dev/pandas/pull/19291/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL19faW5pdF9fLnB5) | | |
| [pandas/core/arrays/categorical.py](https://codecov.io/gh/pandas-dev/pandas/pull/19291/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL2NhdGVnb3JpY2FsLnB5) | | |
| ... and [1 more](https://codecov.io/gh/pandas-dev/pandas/pull/19291/diff?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/19291?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/19291?src=pr&el=footer). Last update [f0cd23c...0115ef1](https://codecov.io/gh/pandas-dev/pandas/pull/19291?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > you basically tripled the code here. pls don't do that. refactor into smaller more logical units.

You complained about the existence of `if/else` conditions in the previous version when `DatetimeBlock` handled both the aware and unaware cases.  Are you suggesting going back to that or something else entirely? i am suggesting a refactor completely of interpolate actually

is very hacky now - this is just making it worse

to be honest i would prob not touch this now

many many other fish to fry OK.  I'll put a pin in this until I can come up with something more elegant.  Mind keeping it open for now so it doesn't fall off my radar screen?