software/slapos-sr-testing: run repman only on python3
in ceb063a4 (software/repman: tests python3 compliance, 2020-10-23), test profile was changed to run on python3 only but this was accidentally reverted in 3a1e5628 (software/slapos-sr-testing: use nxdtest, 2020-10-28), by running repman also on python2. Restore the situation where we run repman only on python3