have _NaT subclass datetime directly
The main change made by this PR is to have `_NaT` subclass `datetime` directly instead of subclassing `_Timestamp`.  Everything else is just to preserve the current behavior.

Why you ask?  Because once this is OKed, the entire `NaT` machinery can be cut/paste into a stand-alone module whose only intra-pandas dependency is `util`.

The only non-trivial behavior is in `__add__` and `__sub__`, where currently `_NaT` dispatches to `_Timestamp` for a few cases.  This PR re-implements those cases directly in `_NaT`.  While that does mean a small amount of repetition, it opens up the possibility of using `@cython.final` for potential optimization in `_Timestamp.__add__/__sub__`.

`NaTType.to_datetime` could use `_make_nat_func` if we were OK with it not producing the same warning as `Timestamp.to_datetime`.

- [ ] closes #17435
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17793?src=pr&el=h1) Report
> Merging [#17793](https://codecov.io/gh/pandas-dev/pandas/pull/17793?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/37860a5fc7c72a3285f060b44482d6d454fa78ae?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/17793/graphs/tree.svg?width=650&src=pr&token=eZ4WkYLtcO&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17793?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17793      +/-   ##
==========================================
- Coverage   91.24%   91.22%   -0.02%     
==========================================
  Files         163      163              
  Lines       49918    49918              
==========================================
- Hits        45546    45537       -9     
- Misses       4372     4381       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.02% <ø> (ø)` | :arrow_up: |
| #single | `40.24% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17793?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17793?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17793?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.73% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17793?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/17793?src=pr&el=footer). Last update [37860a5...d299ab5](https://codecov.io/gh/pandas-dev/pandas/pull/17793?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/17793?src=pr&el=h1) Report
> Merging [#17793](https://codecov.io/gh/pandas-dev/pandas/pull/17793?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/44893893db479894a156dccb28c56f7087b32e14?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/17793/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/17793?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17793      +/-   ##
==========================================
- Coverage   91.24%   91.23%   -0.02%     
==========================================
  Files         163      163              
  Lines       50168    50168              
==========================================
- Hits        45777    45769       -8     
- Misses       4391     4399       +8
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.04% <ø> (ø)` | :arrow_up: |
| #single | `40.27% <ø> (-0.06%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17793?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17793?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17793?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.75% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/indexes/datetimes.py](https://codecov.io/gh/pandas-dev/pandas/pull/17793?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9kYXRldGltZXMucHk=) | `95.5% <0%> (+0.09%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17793?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/17793?src=pr&el=footer). Last update [4489389...6dce07f](https://codecov.io/gh/pandas-dev/pandas/pull/17793?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 going to wait on this till after we release 0.21.0 note that we might as well NOT subclass datetime at all! its a bit tricky though you would have to implement a metaclass so isinstance check would work (IOW you still want this to be a datetime). but that makes this simpler. > its a bit tricky though you would have to implement a metaclass so isinstance check would work 

My impression is that metaclasses in cython are finicky.  That might be out of date.

> going to wait on this till after we release 0.21.0

Sounds good.  In the interim I'll try to finish off the `offsets` and go down the TODO list. i would like to see this not subclass datetime
then this becomes really simple; would still have to be able to masquerade as a datetime/timedelta
for isintsance checks

you can dig into the CPython source to see how this is done > i would like to see this not subclass datetime

I'll put looking into this on the tslibs todo list.

> then this becomes really simple; would still have to be able to masquerade as a datetime/timedelta for isintsance checks

At the moment `isinstance(NaT, Timedelta)` returns False.  Is it supposed to be True? > At the moment isinstance(NaT, Timedelta) returns False. Is it supposed to be True?

hmm maybe we don’t event need to make isinstance be true; so try not subclassing datetime and see what breaks;  > hmm maybe we don’t event need to make isinstance be true; so try not subclassing datetime and see what breaks;

Do you mind saving that for a follow-up?  This has been added to #17652. ok this lgtm. @jbrockmendel if you want to merge in different order that my arbitrary one :> lmk. just ping which one should go first. I'll be happy with any order; go nuts. Way to be a closer today!

Sent from my iPhone

> On Oct 27, 2017, at 5:01 PM, Jeff Reback <notifications@github.com> wrote:
> 
> @jbrockmendel if you want to merge in different order that my arbitrary one :> lmk. just ping which one should go first.
> 
> —
> You are receiving this because you were mentioned.
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> 
 need’s rebase thanks!