Pytest-asyncio

- Fixes a bug that caused event loops to be closed prematurely when using async generator fixtures with class scope or wider in a function-scoped test [708](https://github.com/pytest-dev/pytest-asyncio/issues/708)
- Fixes a bug that caused an internal pytest error when using unittest.SkipTest in a module [711](https://github.com/pytest-dev/pytest-asyncio/issues/711)

- Fixes a bug that caused an internal pytest error when collecting .txt files [703](https://github.com/pytest-dev/pytest-asyncio/issues/703)

- Fixes a bug that caused an internal pytest error when using module-level skips [701](https://github.com/pytest-dev/pytest-asyncio/issues/701)

This release is backwards-compatible with v0.21.
Changes are non-breaking, unless you upgrade from v0.22.

* BREAKING: The _asyncio_event_loop_ mark has been removed. Event loops with class, module, package, and session scopes can be requested via the _scope_ keyword argument to the _asyncio_ mark.
* Introduces the _event_loop_policy_ fixture which allows testing with non-default or multiple event loops [662](https://github.com/pytest-dev/pytest-asyncio/pull/662)
* Removes pytest-trio from the test dependencies [620](https://github.com/pytest-dev/pytest-asyncio/pull/620)