Update Python libraries for GCP services

Some libraries from the GCP requirements of the Apache Beam Python package reference old versions, e.g. Bigtable and Spanner (that had several major version bumps since).

The setup file describes those dependencies as only being required by tests, but it looks like some transforms are referencing them. Therefore I'm not sure of the real cost behind upgrading them.

I'm currently using the Spanner client in a custom transform, but I have to stick to the google-cloud-spanner version used by Apache Beam. Would it be possible to upgrade those dependencies?

Cheers,

Flo