add benchmarks for timestamp parsing
There's a reasonable case to be made that these and timeseries.ToDatetime benchmarks belong in the same place.  LMK if there's a preference.

Small flake8 fixups got timedelta.py passing, enabled linting for modules starting with "t".
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18698?src=pr&el=h1) Report
> Merging [#18698](https://codecov.io/gh/pandas-dev/pandas/pull/18698?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/288bf6e5e3287a8c66a7ed41050db0ebf56d7ea6?src=pr&el=desc) will **decrease** coverage by `0.04%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/18698/graphs/tree.svg?width=650&height=150&src=pr&token=eZ4WkYLtcO)](https://codecov.io/gh/pandas-dev/pandas/pull/18698?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #18698      +/-   ##
==========================================
- Coverage    91.6%   91.56%   -0.05%     
==========================================
  Files         153      153              
  Lines       51272    51272              
==========================================
- Hits        46967    46946      -21     
- Misses       4305     4326      +21
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `89.42% <ø> (-0.03%)` | :arrow_down: |
| #single | `40.68% <ø> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/18698?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/18698/diff?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/18698/diff?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `64.78% <0%> (-1.74%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/18698/diff?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.81% <0%> (-0.1%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/18698?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/18698?src=pr&el=footer). Last update [288bf6e...75e5cc3](https://codecov.io/gh/pandas-dev/pandas/pull/18698?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 thanks!