PERF: regression fixup for timestamp.TimestampConstruction.time_parse_iso8601_tz benchmark
- [x] closes #N/A
- [ ] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`

For detection details see: https://github.com/pandas-dev/pandas/pull/26220#issuecomment-491143978

The slowdown was due to the use of `convert_to_tsobject` function.

Asv result:

master| patch| ratio| test_name
-|-|-|-
31.3±0.5μs|       21.9±0.2μs|     0.70| timestamp.TimestampConstruction.time_parse_iso8601_tz



# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26334?src=pr&el=h1) Report
> Merging [#26334](https://codecov.io/gh/pandas-dev/pandas/pull/26334?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/ee6b1318ff07806ead79dac8119ad9ce580afe1c?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26334/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26334?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26334      +/-   ##
==========================================
- Coverage   92.04%   92.03%   -0.01%     
==========================================
  Files         175      175              
  Lines       52297    52297              
==========================================
- Hits        48137    48134       -3     
- Misses       4160     4163       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.59% <ø> (ø)` | :arrow_up: |
| #single | `40.74% <ø> (-0.13%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26334?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26334/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26334/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.01% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26334/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.7% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26334?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/26334?src=pr&el=footer). Last update [ee6b131...687590f](https://codecov.io/gh/pandas-dev/pandas/pull/26334?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/26334?src=pr&el=h1) Report
> Merging [#26334](https://codecov.io/gh/pandas-dev/pandas/pull/26334?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/681a22c154b84659e90507f7727dbec5cd65d01d?src=pr&el=desc) will **decrease** coverage by `<.01%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/26334/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/26334?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #26334      +/-   ##
==========================================
- Coverage   92.04%   92.03%   -0.01%     
==========================================
  Files         175      175              
  Lines       52292    52292              
==========================================
- Hits        48132    48129       -3     
- Misses       4160     4163       +3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `90.59% <ø> (ø)` | :arrow_up: |
| #single | `40.73% <ø> (-0.13%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/26334?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/26334/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `78.94% <0%> (-10.53%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/26334/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.01% <0%> (-0.12%)` | :arrow_down: |
| [pandas/util/testing.py](https://codecov.io/gh/pandas-dev/pandas/pull/26334/diff?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvdGVzdGluZy5weQ==) | `90.7% <0%> (+0.1%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/26334?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/26334?src=pr&el=footer). Last update [681a22c...40ad4dd](https://codecov.io/gh/pandas-dev/pandas/pull/26334?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @anmyachev : Nice!  Can you show the result of the entire benchmark suite? @gfyoung 
`asv continuous -f 1.05 master HEAD -b ^timeseries -b ^timestamp -a warmup_time=1 -a sample_time=1`:

master| patch| ratio| test_name
-|-|-|-
30.9±0.6μs|       22.1±0.4μs|     0.72| timestamp.TimestampConstruction.time_parse_iso8601_tz @anmyachev : I meant the entire benchmark suite, just in case. this looks fine; waiting on CI resolution. @gfyoung entire benchmark suite will be later. thanks @anmyachev 