check-py3-compat: use an absolute path in sys.path
The idea and rationale is similar to https://phab.mercurial-scm.org/D12599 (landed as 1b6e381521c5).
This instance will be upgraded to Heptapod 1.3.1 (GitLab 16.9.3) on 2023-03-28 at 17:00 UTC+1
The idea and rationale is similar to https://phab.mercurial-scm.org/D12599 (landed as 1b6e381521c5).