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 
Is the error you posted from pip or conda?

Shouldn't the solver know to pick something like 2.8, which is compatible with both? From `pip`. I don't know why the solver doesn't work. It might only work with currently installed packages rather than having collected them all together, before installing them. Anyway, the issue that I duplicated seems to say we should just wait for botocore to fix this, so I'm happy to close this PR. I asked over in https://github.com/boto/botocore/issues/1872 when they think it'll be fixed. i think theres consensus that this needs to be fixed in botocore, not here? Closing, needs fix in botocore.  If I'm wrong on this, please correct me and re-open.