there is chance that the distro install python as python3, in that case
the shebang of
```
```
in build.py won't work.
Signed-off-by: Kefu Chai <tchaikov@gmail.com>
this allows us to use sphinx-build-3, as user might already in a python3
world, and distros, like fedora 28, tend to add a "-3" suffix to sphinx-build.
Signed-off-by: Kefu Chai <tchaikov@gmail.com>
Kefu Chai