xcube cannot access geotiff datasets from public buckets

[TonioF]

To Reproduce Steps to reproduce the behavior:

1. Create an xcube store to geotiff datasets stored in a public s3 bucket (e.g., from here: https://registry.opendata.aws/copernicus-dem/ )
2. Open one of the datasets (ds = store.open_data(...) )
3. Try to open the base_dataset: ds.base_dataset
4. You'll receive this error: AttributeError: 'NoneType' object has no attribute 'Session'

It seems this error only occurs when boto3 is not installed. Also, it started appearing with xcube 1.1.2.

[pont-us]

Confirmed with a fresh xcube 1.2.0 install from conda-forge and the following code.

import xcube.core.store

datastore = xcube.core.store.new_data_store(
    's3',
    root='copernicus-dem-30m'
)

dataset = datastore.open_data(
    'Copernicus_DSM_COG_10_S15_00_E143_00_DEM/'
    'Copernicus_DSM_COG_10_S15_00_E143_00_DEM.tif'
)

print(dataset.base_dataset)