2.1.0 - Internal engine improvements + bugfixes

Fixed issue with @parametrize_with_cases when two cases with the same id and both requiring a fixture were to be created. Fixed #117.

Fixture closure engine refactoring:

• When no fixture unions are present, the fixture closure is now identical to the default one in pytest, to avoid issues originating from other plugins fiddling with the closure. Fixes #116

• New SuperClosure class representing the "list" facade on top of the fixture tree (instead of FixtureClosureNode). In addition, this list facade now better handles editing the order of fixtures when possible. Fixes #111.

• Session and Module-scoped fixtures that are not used in all union alternatives are not any more torn town/setup across union alternatives. Fixes #120

See documentation page for details.