Force usage of python3

The PYTHON variable in the server might still be python(2)
8 jobs for debian in 9 minutes and 25 seconds (queued for 3 seconds)
latest
Status Job ID Name Coverage
  Build
passed #328817
build

00:03:28

 
  Test
passed #328822
autopkgtest

00:04:02

passed #328823
blhc

00:02:08

passed #328821
lintian

00:02:13

passed #328824
piuparts

00:05:36

passed #328820
reprotest

00:03:06

passed #328819
test-build-all

00:02:55

passed #328818
test-build-any

00:03:50