v0.3.4: PyPI publishing, for real

Fixed

• PyPI publishing, for real. 0.3.3 moved the upload into a job of this
  repository's release.yml but wrapped pypa/gh-action-pypi-publish in a
  composite action, and that action names its Docker image from the
  repository that contains it, so the image pull failed. The pypa action is
  now a direct step of the job, as it was for 0.2.0. 0.3.4 carries the code of
  0.3.1 to 0.3.3 unchanged and is the first 0.3 release on PyPI.