Bug: Allow np.timedelta64 objects to index TimedeltaIndex
- [x] closes #20393
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

`is_integer(label)` would catch `np.timedelta64` objects due to https://github.com/numpy/numpy/issues/10685 (I believe), so ensure that the label is also not a timedelta64 dtype
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20408?src=pr&el=h1) Report
> Merging [#20408](https://codecov.io/gh/pandas-dev/pandas/pull/20408?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7d5e65343060eb21d81422cc1106b4d90305d8df?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/20408/graphs/tree.svg?height=150&width=650&token=eZ4WkYLtcO&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/20408?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #20408   +/-   ##
=======================================
  Coverage    91.8%    91.8%           
=======================================
  Files         152      152           
  Lines       49205    49205           
=======================================
  Hits        45171    45171           
  Misses       4034     4034
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.18% <100%> (ø)` | :arrow_up: |
| #single | `41.85% <0%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/20408?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/20408/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `91.22% <100%> (ø)` | :arrow_up: |

------

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> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/20408?src=pr&el=footer). Last update [7d5e653...199e819](https://codecov.io/gh/pandas-dev/pandas/pull/20408?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @mroeschke  thanks!