devel/py-pytest-mock: Update to 3.15.1

Tweaks on top of the PR patch:

• Use DISTVERSION (referenced by ${DISTVERSION} in DISTNAME) instead of PORTVERSION.
• Add py-wheel to BUILD_DEPENDS (required by USE_PYTHON=pep517 with the setuptools build backend).
• Bump pytest dep to >=6.2.5 to match upstream pyproject.toml.

PR: 273337
Submitted by: takefu