Six tests in the local subprocess and parallel suites fail on the weekly
Python 3.15 check (run
[25667044284](https://github.com/tox-dev/tox/actions/runs/25667044284))
because every spawned interpreter prints a `KeyError` to stderr before
user code runs, and the assertions compare stderr byte-for-byte.

The noise originates in the legacy `*-nspkg.pth` files shipped by
`PasteDeploy 3.1.0` and `repoze.lru 0.7`, both pulled in transitively by
`devpi-server` → `pyramid`. Those pth files call
`sys._getframe(1).f_locals['sitedir']`, which worked when `site.py`
walked its own frame. PEP 829 (CPython
[#149109](python/cpython#149109)) replaced that
machinery with a deferred `_exec_imports()` that has no `sitedir` local,
so every interpreter startup raises `KeyError: "local variable
''sitedir'' is not defined"`. The regression is tracked upstream in
CPython [#149671](python/cpython#149671).

This is genuinely an upstream problem: the proper fix lives either in
CPython (restoring compatibility with legacy `.pth` hacks, or providing
a migration path) or in `setuptools` (which still generates these pth
files for `namespace_packages=` declarations even though `pkg_resources`
was removed in 82.0.0). The `paste` and `repoze` namespaces would still
resolve via PEP 420 without the `.pth` shim, so the affected packages
could also drop `namespace_packages=` upstream.

Until any of those land, this patch removes the offending pth files at
conftest import — gated by the broken `f_locals['sitedir']` substring so
unrelated namespace pth files are untouched, and wrapped in
`contextlib.suppress(OSError)` to stay idempotent under `xdist` workers
racing on the same file. Revert once the upstream fix is available.