tests/conftest.py

import pytest

def pytest_addoption(parser):
    parser.addoption("--runslow", action="store_true", default=False)

def pytest_configure(config):
    config.addinivalue_line("markers", "slow: slow convergence tests")

def pytest_collection_modifyitems(config, items):
    if config.getoption("--runslow"):
        return
    skip = pytest.mark.skip(reason="need --runslow")
    for item in items:
        if "slow" in item.keywords:
            item.add_marker(skip)