xcube is not compatible with Python 3.10

[TonioF]

Since the release of Python 3.10.2 on January 14th, 2022, xcube builds fail with errors like:

ImportError: cannot import name 'MutableMapping' from 'collections'

This is because abstract base classes have been moved from Python's 'collections' package to 'collections.abc'. While there are a few instances in the xcube code where the import needs to be adjusted, this issue also affects the pydap package, which is a transitive dependency of xarray and is beyond our fixing scope.

Also, the relevant change has already been included in Python 3.10, for some reason, Python 3.9 was used for xcube builds until the release of Python 3.10.2.

[TonioF]

As an immediate solution, we'll pin xcube to use python versions < 3.9 and remove faulty imports in our code.

[TonioF]

The immediate fix above was applied. We'll leave this issue open until xcube is compatible with Python 3.10