- Revive asciidoc; will be replaced or dropped later, depending on
the general solution of asciidoc support on TW

- Update to 0.21:
* Build reproducibility repairs (Thanks reproducible-builds.org team!)
* Faster handling of large patches (#44)
* Python can now be run with optimizations (python -O)
* stg log now prints trailing newline
* Improved command line option parsing for stg log

- Switch to python3 to build
- Disable documentation generating using asciidoc as that requires
python2