build: Don't hardcode the path of python3

Hardcoding paths to interpreters seldom works on *BSD. The ports system
allows users to choose their preferred installation prefix as long as it
is not /usr. This makes it nearly impossible to have a common path of
python3 which works on both GNU/Linux and *BSD.
3 jobs for wip/lantw/dont-hard-code-usr-bin-python3 in 2 minutes and 43 seconds
Status Job ID Name Coverage
  Test
passed #26502
ubuntu-16.04

00:02:20

passed #26503
ubuntu-17.10

00:02:07

passed #26504
ubuntu-18.04

00:02:43