fix release tox env

Author: graingert

pyproject.toml

@@ -86,6 +86,8 @@ legacy_tox_ini = """
  deps =
  build
  twine
+ httpbin @ https://github.com/graingert/httpbin/archive/refs/heads/patch-1.zip
+ cffi >= 1.17.0rc1
  commands =
  {envpython} -m release
  pyproject-build --sdist