CLN: Use public method to capture UTC offsets
xref https://github.com/pandas-dev/pandas/pull/21822#discussion_r207023121

Per @pganssle's suggestion, avoiding using private dateutil methods to access the timezone offset and more succinctly using `utcoffset` instead

Does this mean instead of capturing eg US/Pacific we get -0800? (Never mind that date until doesn’t yet parse US/Pacific) Essentially any datetime string with some sort of timezone information (that gets successfully parsed) will be returned as a fixed offset. # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22164?src=pr&el=h1) Report
> Merging [#22164](https://codecov.io/gh/pandas-dev/pandas/pull/22164?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/647f3f091eedd715b7966829a36ef345de4d7a4d?src=pr&el=desc) will **not change** coverage.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/22164/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/22164?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #22164   +/-   ##
=======================================
  Coverage   92.06%   92.06%           
=======================================
  Files         170      170           
  Lines       50720    50720           
=======================================
  Hits        46694    46694           
  Misses       4026     4026
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.47% <ø> (ø)` | :arrow_up: |
| #single | `42.29% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/22164?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/22164?src=pr&el=footer). Last update [647f3f0...f6755fa](https://codecov.io/gh/pandas-dev/pandas/pull/22164?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks @mroeschke and @pganssle 