Blender Git Loki

Manpage gen script: fix for python < 3.7, and missing Blender build info.

Python3.7 is still fairly recent, not all distro use it as system python
yet, fallback to code compatible up to py3.5.

Also, often distro builds of Blender do not have the buildinfo, in that
case fallback to `SOURCE_DATE_EPOCH` envvar, and as last resort to
current time, as in orig patch D5756 (we still use blender builddate
when available).

Issues raised in recent own rBcd5c70630318.