BUG: fix Series[timedelta64] arithmetic with Timedelta scalars
closes #18846 

Before:

```
>>> ser = pd.Series([pd.Timedelta(minutes=5, seconds=3)] * 3)
>>> ser.iloc[-1] = np.nan
>>> scalar = pd.Timedelta(minutes=5, seconds=4)
>>> ser // scalar
[...]
TypeError: can only operate on a timedeltas for addition, subtraction, and division, but the operator [__floordiv__] was passed

>>> scalar / ser
0   00:00:00.000000
1   00:00:00.000000
2               NaT
dtype: timedelta64[ns]

>>> scalar.to_pytimedelta() / ser
0   00:00:00.000000
1   00:00:00.000000
2               NaT
dtype: timedelta64[ns]

>>> scalar.to_timedelta64() / ser
0   00:00:00.000000
1   00:00:00.000000
2               NaT
dtype: timedelta64[ns]

>>> scalar.to_pytimedelta() // ser
[...]
TypeError: can only operate on a timedeltas for addition, subtraction, and division, but the operator [__rfloordiv__] was passed

>>> scalar.to_timedelta64() // ser
[...]
TypeError: can only operate on a timedeltas for addition, subtraction, and division, but the operator [__rfloordiv__] was passed

```

After
```
>>> ser // scalar
0    0.0
1    0.0
2    NaN
dtype: float64

>>> scalar / ser
0    1.0033
1    1.0033
2       NaN
dtype: float64

>>> scalar.to_pytimedelta() / ser
0    1.0033
1    1.0033
2       NaN
dtype: float64

>>> scalar.to_timedelta64() / ser
0    1.0033
1    1.0033
2       NaN
dtype: float64

>>> scalar.to_pytimedelta() // ser
0    1.0
1    1.0
2    NaN
dtype: float64

>>> scalar.to_timedelta64() // ser
0    1.0
1    1.0
2    NaN
dtype: float64

```

Also identified a bug in `Timedelta.__floordiv__`, noted it in #18824, will address that separately.

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

Travis timeout # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18831?src=pr&el=h1) Report
> Merging [#18831](https://codecov.io/gh/pandas-dev/pandas/pull/18831?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/21f2412cf1c3bcbde04c1a53e6ecc597b5e871dd?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18831/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/18831?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18831      +/-   ##
==========================================
- Coverage   91.58%   91.58%   -0.01%     
==========================================
  Files         150      150              
  Lines       48972    48972              
==========================================
- Hits        44851    44849       -2     
- Misses       4121     4123       +2
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.94% <ø> (-0.01%)` | :arrow_down: |
| #single | `41.72% <ø> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18831?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/ops.py](https://codecov.io/gh/pandas-dev/pandas/pull/18831/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvb3BzLnB5) | `90.24% <ø> (ø)` | :arrow_up: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/18831/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `84.74% <0%> (-0.22%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18831?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/18831?src=pr&el=footer). Last update [21f2412...729d240](https://codecov.io/gh/pandas-dev/pandas/pull/18831?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 I think this, #18877, #18875, #18832, and #18783 have all made it through the gauntlet.  #18884 and #18762 need clarification on the requested reivisions.  Everything else needs attention and will hopefully get it over the weekend. Ping Looks like a test got lost in the last merge, will need to track that down.  If there's a way to cancel the CI run, go for it. Merge/rebase issue has been resolved.  This should be good to go (along with #18849 and #18883). thanks!