Add Google Cloud Storage support; deprecate python < 3.8

Contents

New Features

• Add GoogleCloudStorageClient
• Write manifest, its datasets, and its datafiles to cloud in Analysis.finalise (#96)
• Closes #84 - add auto tag and release workflow
• Allow Google Cloud storage blobs to be represented by Pathable
• Add Datafile, Dataset, and Manifest to_cloud and from_cloud methods
• Allow regular GCP files to be represented as Datafiles

Minor fixes and improvements

• Add cloud storage emulator once for all tests
• Add disk usage and file age utilities
• Allow Datasets to have custom names
• Add storage.path module akin to os.path but for Google Cloud Storage paths
• Allow Hashables' hash values to be set
• Pass GCP project and bucket names to tests from environment (#93)
• Add ability to delete topic and subscription when a Service has finished serving
• Facilitate graceful exit for serving Services on KeyboardInterrupt
• Use latest versions of flake8, isort, and black in pre-commit and across all files (#87)
• Fix CI test skipping flag
• Fix documentation links (#92)

Breaking changes

• Remove testing and explicit support for python3.6 and python3.7
• Remove base_from from Pathable and replace with more transparent method
• Rename Datafile.posix_timestamp to Datafile.timestamp and remove default value
• Make Datafile.last_modified private
• Rename persistence subpackage to storage

Testing

• Test that children can question their own children as part of answering a question
• Close #94 - delete topics and subscriptions at the end of each test
• Remove timeouts from tests and replace with thread executor shutdown upon test pass, meaning that tests that connect to Google Pub/Sub won't fail because the connection is slower than expected