PYTHON-3968 Fix mockupdb tox config

[ShaneHarvey]

Fixes:

[2023/10/31 23:28:46.782] test-mockupdb: commands[1]> .evergreen/check-c-extensions.sh
[2023/10/31 23:28:46.783] test-mockupdb: failed with .evergreen/check-c-extensions.sh (resolves to .evergreen/check-c-extensions.sh) is not allowed, use allowlist_externals to allow it
[2023/10/31 23:28:46.783] .pkg: _exit> python /opt/python/3.7/lib/python3.7/site-packages/pyproject_api/_backend.py True setuptools.build_meta
[2023/10/31 23:28:46.812]   test-mockupdb: FAIL code 1 (13.87 seconds)
[2023/10/31 23:28:46.812]   evaluation failed :( (13.97 seconds)

[blink1073]

LGTM!

[ShaneHarvey]

posargs is not needed here because its already added via {[testenv:test]commands}. Having it here actually duplicates the args, eg see how --help becomes --help --help:

 python -m tox -e test-mockupdb -- --help           
.pkg: _optional_hooks> python /Users/shane/work/pycharm/pymongo-py311/lib/python3.11/site-packages/pyproject_api/_backend.py True setuptools.build_meta
.pkg: get_requires_for_build_sdist> python /Users/shane/work/pycharm/pymongo-py311/lib/python3.11/site-packages/pyproject_api/_backend.py True setuptools.build_meta
.pkg: build_sdist> python /Users/shane/work/pycharm/pymongo-py311/lib/python3.11/site-packages/pyproject_api/_backend.py True setuptools.build_meta
test-mockupdb: install_package> python -I -m pip install --force-reinstall --no-deps /Users/shane/git/mongo-python-driver/.tox/.tmp/package/73/pymongo-4.6.0.dev0.tar.gz
test-mockupdb: commands[0]> python --version
Python 3.11.4
test-mockupdb: commands[1]> .evergreen/check-c-extensions.sh
running build_ext
test-mockupdb: commands[2]> pytest -v --durations=5 --maxfail=10 --help --help ./test/mockupdb
usage: pytest [options] [file_or_dir] [file_or_dir] [...]