Support pytest 9.2 #146

With pytest 9.2 and pytest-benchmark installed, pytest --codspeed crashes at session start:

AttributeError: 'FixtureManager' object has no attribute '_arg2fixturedefs'

pytest 9.2 replaced the private FixtureManager._arg2fixturedefs attribute with _arg2node2fixturedefs (pytest-dev/pytest#14984, fixing pytest-dev/pytest#14942). pytest-codspeed writes into that attribute to override pytest-benchmark's benchmark fixture.

Workaround: pin pytest<9.2 until a fixed release is out.

Fix in progress: #141