pytest: Fix deprecation warning when running against pytest 5.4

See
https://docs.pytest.org/en/stable/deprecations.html#node-construction-changed-to-node-from-parent

stbt_rig.py

@@ -1223,7 +1223,13 @@ def pytest_addoption(parser):
 
     def pytest_collect_file(path, parent):
         if path.ext == ".py":
-            return StbtCollector(path, parent)
+            if hasattr(StbtCollector, "from_parent"):
+                # pytest >v5.4
+                return StbtCollector.from_parent(parent=parent, fspath=path)
+            else:
+                # Backwards compat
+                # https://docs.pytest.org/en/stable/deprecations.html#node-construction-changed-to-node-from-parent
+                return StbtCollector(path, parent)
         else:
             return None