TST: Test against python-dateutil master
https://github.com/dateutil/dateutil/issues/397

just for testing
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16648?src=pr&el=h1) Report
> Merging [#16648](https://codecov.io/gh/pandas-dev/pandas/pull/16648?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9fdea65e5675f724efbb6afd68b5862128aa388d?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/16648/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16648?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16648   +/-   ##
=======================================
  Coverage   90.93%   90.93%           
=======================================
  Files         161      161           
  Lines       49265    49265           
=======================================
  Hits        44800    44800           
  Misses       4465     4465
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.69% <ø> (ø)` | :arrow_up: |
| #single | `40.22% <ø> (ø)` | :arrow_up: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16648?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/16648?src=pr&el=footer). Last update [9fdea65...35c303e](https://codecov.io/gh/pandas-dev/pandas/pull/16648?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/16648?src=pr&el=h1) Report
> Merging [#16648](https://codecov.io/gh/pandas-dev/pandas/pull/16648?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2e24a8f8886a1481af8324d44224cddb2ee1b53c?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/16648/graphs/tree.svg?width=650&token=eZ4WkYLtcO&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16648?src=pr&el=tree)

```diff
@@           Coverage Diff           @@
##           master   #16648   +/-   ##
=======================================
  Coverage   90.91%   90.91%           
=======================================
  Files         161      161           
  Lines       49284    49284           
=======================================
  Hits        44809    44809           
  Misses       4475     4475
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.69% <ø> (ø)` | :arrow_up: |
| #single | `40.17% <ø> (-0.01%)` | :arrow_down: |


------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16648?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/16648?src=pr&el=footer). Last update [2e24a8f...69e7369](https://codecov.io/gh/pandas-dev/pandas/pull/16648?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 updated to use master. let's see how this does.