py3: fix broken man page generation, it was generating `(default: NUL*)` `bytes(default)` was producing things like `(default: \x00)` when handed non-bytes values such as `1`, `10`, or `True`. The man page generation would apparently ignore these bytes and produce man pages that had the string `(default: )`. Test Plan: - Ran `cd doc; python3 gendoc.py "hg.1.gendoc"` and grepped for bad output - Ran `make deb`, extracted the deb, manually inspected `hg.1` file. Differential Revision: https://phab.mercurial-scm.org/D8639