pytest: replace existing process

[stefanv]

@mattip Is this enough to address your gdb issue, or do I need to try something a bit smarter?

[mattip]

LGTM, thanks.

Details about how I verified it works

From a clean checkout of numpy, before that change:

$ python3.9 -m venv /tmp/venv3
$ . /tmp/venv3/bin/activate
$ python -m pip install -r build_requirements.txt
$ python -m pip install -r test_requirements.txt
$ python dev.py build -- --debug
$ gdb --args python dev.py test -- numpy/core/tests/test_ufunc.py
(gdb) b ufunc_at
Function "ufunc_at" not defined.
Make breakpoint pending on future shared library load? (y or [n]) y
Breakpoint 1 (ufunc_at) pending.
(gdb) r
Starting program: /tmp/venv3/bin/python dev.py test -- numpy/core/tests/test_ufunc.py
...
# tests pass without hitting the breakpoint

Running the gdb sesion after the change, I hit the breakpoint and can debug.