make collection strategy={None, "api"} selectable when run via pytest

[ev-br]

The doctest layer allows the choice of strategy={None, "api"}. The pytest layer hardcodes "api". Would be nice to make it selectable in the plugin, too.

[ev-br]

This is needed for dev.py -t _impl/file: otherwise all is deselected.

Sadly, this is only testable in a package, as pytest -t path/to/file just works in seld-tests. That make sense: self-tests run on scpdt itself, and it does not have the public/private split.