Configuration option for default loop scope

[seifertm]

When a user wants to run all tests in a session-scoped event loop, the documentation suggests implementing a pytest_collection_modifyitems hook.

In auto mode the hook causes duplicate asyncio markers to be attached to test items. This is a cosmetic issue, but creates additional noise when debugging item markers.

In strict mode the behavior of the hook rules out manual overrides of the asyncio mark.

A configuration option for the default loop scope as suggested in #705 (comment) would eliminate duplicate asyncio markers in auto mode and allow overriding the default loop scope with explicit markers in strict mode.