cdef out dtype for _Timestamp._get_field
Follow-up to @jreback [comment](https://github.com/pandas-dev/pandas/pull/17377#discussion_r136948947) in #17377 to dtype the `out` variable in `_get_field` and `get_named_field`

Also, replaced value conversion in `_get_start_end_field` with `_maybe_convert_value_to_local` function created in #17377
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17457?src=pr&el=h1) Report
> Merging [#17457](https://codecov.io/gh/pandas-dev/pandas/pull/17457?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/20fee85ede7f2052f855b8f3445cd1ffc17ee0c3?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/17457/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/17457?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #17457      +/-   ##
==========================================
- Coverage   91.15%   91.14%   -0.02%     
==========================================
  Files         163      163              
  Lines       49591    49591              
==========================================
- Hits        45207    45198       -9     
- Misses       4384     4393       +9
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.92% <ø> (ø)` | :arrow_up: |
| #single | `40.25% <ø> (-0.07%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/17457?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/17457?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/17457?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.72% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/17457?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/17457?src=pr&el=footer). Last update [20fee85...52a5f98](https://codecov.io/gh/pandas-dev/pandas/pull/17457?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!