Fix AST nodes for Python 3 and enable dependent test_uri (#18597)

* Enable tests on python 3 for uri

* Added one more node type to SAFE_NODES into safe_eval module.

ast.USub represents unary operators. This is necessary for
parsing some unusual but still valid JSON files during testing
with Python 3.

test/utils/shippable/python3-test-tag-blacklist.txt

@@ -1,3 +1,2 @@
 test_hg
 test_service
-test_uri