use Timedelta instead of convert_to_timedelta64
- [ ] closes #xxxx
- [ ] tests added / passed
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24821?src=pr&el=h1) Report
> Merging [#24821](https://codecov.io/gh/pandas-dev/pandas/pull/24821?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/512830b46e678a1c6bd86cc60f54d74ff75792db?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24821/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24821?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24821      +/-   ##
==========================================
- Coverage   92.38%   92.38%   -0.01%     
==========================================
  Files         166      166              
  Lines       52379    52378       -1     
==========================================
- Hits        48392    48390       -2     
- Misses       3987     3988       +1
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.91% <25%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24821?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/tools/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/24821/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvdGltZWRlbHRhcy5weQ==) | `98% <100%> (-0.04%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/24821/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `88.04% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24821?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/24821?src=pr&el=footer). Last update [512830b...d1bc2c1](https://codecov.io/gh/pandas-dev/pandas/pull/24821?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/24821?src=pr&el=h1) Report
> Merging [#24821](https://codecov.io/gh/pandas-dev/pandas/pull/24821?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2bbe418409d88f60daeb9e12e738cf8c9633eccc?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/24821/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/24821?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #24821      +/-   ##
==========================================
- Coverage   92.38%   92.38%   -0.01%     
==========================================
  Files         166      166              
  Lines       52379    52378       -1     
==========================================
- Hits        48392    48391       -1     
  Misses       3987     3987
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.81% <100%> (-0.01%)` | :arrow_down: |
| #single | `42.91% <25%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/24821?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/tools/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/24821/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvdG9vbHMvdGltZWRlbHRhcy5weQ==) | `98% <100%> (-0.04%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/24821?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/24821?src=pr&el=footer). Last update [2bbe418...8c6d63e](https://codecov.io/gh/pandas-dev/pandas/pull/24821?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 > prob can now change convert_to_timedelta64 to cdef from cpdef

Good idea.  Also removed some util.is_array checks that can never be True. thanks!