Dcoraid

- setup: drop support for Python 3.9
- ci: migrate from appveyor to GitHub Actions

- fix: twisted error message upon SHA-256 upload verification (73)
- fix: user_show raised unexpected APINotFoundError in wizard
- fix: support None as empty API key
- fix: explain to user how to create API token, since CKAN 2.10 does
not support automatic generation of API keys anymore
- fix: workaround for broken CKAN logic in user_show
- enh: skip SSL verification via DCORAID_SKIP_SSL_VERIFY environment variable
- enh: add ``__repr__`` for ``CKANAPI``
- ref: introduce `CKANAPI.ckan_version_object` fetching the CKAN version

- fix: set row count to zero when all jobs are removed from upload queue
- enh: increase buffer size for uploads to 4MB

- setup: bump requests to 2.31.0 (CVE-2023-32681)
- setup: bump dclab from 0.50.2 to 0.52.0

- fix: properly propagate SystemExit exit codes

- fix: make CACHE_DIR an optional argument to dclab-upload-task