Log Message:
asciidoc: update to 9.0.4.

Version 9.0.4 (2020-10-20)
--------------------------
.Bug fixes
- Fix listing out installed plugins (e.g. --filter list)
- Fix python version check failing on 3.10 (thanks @hoadlck)

.Testing
- Update to deadsnakes/python@v2.0.0 for testing dev python versions
- Move from testing against 3.9-dev to stable 3.9
- Add 3.10-dev test target