BUILD: Prefer python3 for intgcheck
    
    Samba 4.11.0 dropped support for python2 and thus it was also
    dropped from samba related libraries (ldb ...)
    which is required by integration tests
    
    Merges: https://pagure.io/SSSD/sssd/pull-request/4075
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/4074
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>\
    
    Reviewed-by: Tomáš Halman <thalman@redhat.com>