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