Make "--pdb" imply "--dist no"

[adamchainz]

Fixes #549.

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[blueyed]

Maybe relevant: #502

[adamchainz]

Yes, will happily rebase if that gets merged.

[adamchainz]

Tests are now passing since master got fixed.

[adamchainz]

Thanks for the update

[nicoddemus]

Thanks a lot @adamchainz! 🙇