textproc/py-docstring_parser: update to 0.16

While here:

• switch to USE_PYTHON=pep517
• switch to USE_PYTHON=pytest and remove TEST_DEPENDS and do-test target

Changelog: https://github.com/rr-/docstring_parser/blob/0.16/CHANGELOG.md

PR: 286110
Reviewed by: kai
Approved by: kai