v0.20.0 Release

Storage Changes

• feat: implement resumable parallel file uploads (#3389) - allow applications
  to use multiple threads to upload large files, achieving throughput in excess
  of 1GiB/s. With this change applications can restart such uploads even if the
  application restarts.
• chore: upgrade testbench to Python 3 (#3402) - you will need to have Python 3
  installed to run the integration tests.

Bigtable Changes

• No changes to the Bigtable client in this release.