Implement TimedeltaArray._from_sequence
and have both `TimedeltaArray.__new__` and `TimedeltaIndex.__new__` dispatch to it

One test will fail until #23789 is merged.

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

Hello @jbrockmendel! Thanks for submitting the PR.

 - There are no PEP8 issues in the file [`pandas/core/arrays/timedeltas.py`](https://github.com/jbrockmendel/pandas/blob/b770d7b2b2147b232e7e14a0807405ab98718df6/pandas/core/arrays/timedeltas.py) !

 - There are no PEP8 issues in the file [`pandas/core/indexes/timedeltas.py`](https://github.com/jbrockmendel/pandas/blob/b770d7b2b2147b232e7e14a0807405ab98718df6/pandas/core/indexes/timedeltas.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arithmetic/test_timedelta64.py`](https://github.com/jbrockmendel/pandas/blob/b770d7b2b2147b232e7e14a0807405ab98718df6/pandas/tests/arithmetic/test_timedelta64.py) !

 - There are no PEP8 issues in the file [`pandas/tests/arrays/test_timedeltas.py`](https://github.com/jbrockmendel/pandas/blob/b770d7b2b2147b232e7e14a0807405ab98718df6/pandas/tests/arrays/test_timedeltas.py) !

 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23953?src=pr&el=h1) Report
> Merging [#23953](https://codecov.io/gh/pandas-dev/pandas/pull/23953?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/7653a6b43a8a302bc1d88903c8c99730a00bcc80?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/23953/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/23953?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #23953      +/-   ##
==========================================
+ Coverage    92.3%   92.31%   +<.01%     
==========================================
  Files         161      161              
  Lines       51555    51547       -8     
==========================================
- Hits        47589    47584       -5     
+ Misses       3966     3963       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.7% <100%> (ø)` | :arrow_up: |
| #single | `42.44% <85.71%> (ø)` | :arrow_up: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/23953?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/arrays/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/23953/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvYXJyYXlzL3RpbWVkZWx0YXMucHk=) | `96% <100%> (+0.04%)` | :arrow_up: |
| [pandas/core/indexes/timedeltas.py](https://codecov.io/gh/pandas-dev/pandas/pull/23953/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy90aW1lZGVsdGFzLnB5) | `90.11% <100%> (+0.5%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/23953?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/23953?src=pr&el=footer). Last update [7653a6b...4a60119](https://codecov.io/gh/pandas-dev/pandas/pull/23953?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 can you rebase rebased+green thanks