devel/py-virtualenv: Update to 20.26.3

• Remove pre-test target
• Remove files/patch-pyproject
• Update test dependencies

Release changes:

https://github.com/pypa/virtualenv/releases/tag/20.26.3
https://github.com/pypa/virtualenv/releases/tag/20.26.2

Reported by: Bernát Gábor <notifications@github.com>