[RFC] --pdb: Friendlier default (use warning and infer --reruns 0)

[tony]

Assume pyproject.toml:

[tool.pytest.ini_options]
addopts = "--reruns 2"

Current behavior

Right now, passing --pdb will raise:

ERROR: --reruns incompatible with --pdb

Better: Assume --reruns 0 if --pdb

It's highly unlikely a person attempting to drop into a shell wants reruns. Rather than error, show a warning:

WARNING: --reruns incompatible with --pdb, --reruns will behave as if set to 0