`ERROR: Double requirement given: `pytest>=6.0.0` (already in pytest, name='pytest')

[NiklasRosenstein]

This occurs when creating a Python 3.8 virtual environment and using slap install to install kraken-core. It has an optional runtime-dependency on pytest and a non-optional development dependency on it.

[tool.poetry.dependencies]
pytest = { version = ">=6.0.0", optional = true }  # For the testing fixture provided by kraken.core

[tool.poetry.dev-dependencies]
pytest = "*"

Upgrading Pip in the virtual environment makes it work.

.venvs/3.8/bin/pip install --upgrade pip