This change ports Ic8936c2e657a2a0212b1bc96915ba6e2035569b3 from
nova's specs repo to fix the redirect generation
The type of app.builder.srcdir is now a _StrPath which
breaks the invocation of replace when generating
redirects.
This change just fixes that by wrapping it in str()
Change-Id: I123aec140d9871ad27e522c107edad40efaf2a19