pytest: compile Yul with --target-version shanghai when fork is cancun

tests/conftest.py

@@ -0,0 +1,20 @@
+"""
+Pytest (plugin) definitions local to Cancun tests.
+"""
+import warnings
+
+import pytest
+
+
+def pytest_collection_modifyitems(items):
+    """
+    Modify tests post collection.
+
+    Here we override the default behavior of the `yul` fixture so that
+    solc compiles with shanghai instead of cancun (which is unavailable
+    in solc 0.8.20).
+    """
+    for item in items:
+        if "Cancun" in item.name and "yul" in item.fixturenames:
+            warnings.warn("Compiling Yul source with Shanghai, not Cancun.")
+            item.add_marker(pytest.mark.compile_yul_with("Shanghai"))