Add switch to use Python 3 instead of Python 2

by setting GAUDI_USE_PYTHON_MAJOR
11 jobs for cmake-modernisation in 29 minutes and 44 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #8919104
cvmfs
lhcb-gcc9

00:02:58

passed #8919103
cvmfs
view-gcc8

00:01:28

passed #8919102
cvmfs
x86_64-centos7-gcc9-dbg

00:06:29

passed #8919101
cvmfs
x86_64-centos7-gcc9-opt

00:02:53

 
  Test
passed #8919238
cvmfs
lhcb-gcc9:test

00:05:37

passed #8919107
cvmfs
view-gcc8:test

00:04:31

passed #8919272
cvmfs
x86_64-centos7-gcc9-dbg:test

00:09:22

passed #8919105
cvmfs
x86_64-centos7-gcc9-opt:test

00:08:14

failed #8919108
cvmfs
lhcb-gcc9:test

00:12:28

failed #8919201
cvmfs
x86_64-centos7-gcc9-dbg:test

00:12:36

failed #8919106
cvmfs
x86_64-centos7-gcc9-dbg:test

00:11:36