py3: use '%d' for integers rather than '%s'
obsolete._readmarkers() returns an integer version number.
Differential Revision: https://phab.mercurial-scm.org/D1537
## Avoid interference from actual test env:unset HGTEST_JOBSunset HGTEST_TIMEOUTunset HGTEST_PORTunset HGTEST_SHELL