Pin dateutil to 2.8.0 in requirements
botocore has pinned dateutil to <=2.8.0, which conflicts with our dateutil here and leads to 

```
ERROR: botocore 1.13.12 has requirement python-dateutil<2.8.1,>=2.1; python_version >= "2.7", but you'll have python-dateutil 2.8.1 which is incompatible.
```

See https://github.com/boto/botocore/commit/e87e7a745fd972815b235a9ee685232745aa94f9

Closes #29465 