tox: allow passing multiple extra pytest arguments

Example: `tox -e py311-unit -- --lf --pdb`

Author: mr-c

tox.ini

@@ -66,10 +66,10 @@ commands_pre =
   py312-lintreadme: python -m build --outdir {distdir}
 
 commands =
-  py3{9,10,11,12,13}-unit: make coverage-report coverage.xml PYTEST_EXTRA={posargs}
+  py3{9,10,11,12,13}-unit: make coverage-report coverage.xml PYTEST_EXTRA="{posargs}"
   py3{9,10,11,12,13}-bandit: bandit -r cwltool
   py3{9,10,11,12,13}-lint: make flake8 format-check codespell-check
-  py3{9,10,11,12,13}-mypy: make mypy mypyc PYTEST_EXTRA={posargs}
+  py3{9,10,11,12,13}-mypy: make mypy mypyc PYTEST_EXTRA="{posargs}"
   py312-shellcheck: make shellcheck
   py312-pydocstyle: make diff_pydocstyle_report
   py312-lintreadme: twine check {distdir}/*