feat(order_utils.py): schema resolver cache (#1317)

* Implemented basic functionality for using cache layer of LocalRefResolver

* Use `importlib` instead of `imp`, since it's been deprecated. Legacy `load_module()` reloads modules even if they are already imported, causing tests to fail when run in non-deterministic ordering, so we replace it with `import_module()`

python-packages/order_utils/test/test_json_schemas.py

@@ -0,0 +1,23 @@
+"""Tests of zero_ex.json_schemas"""
+
+
+from zero_ex.order_utils import make_empty_order
+from zero_ex.json_schemas import _LOCAL_RESOLVER, assert_valid
+
+
+def test_assert_valid_caches_resources():
+    """Test that the JSON ref resolver in `assert_valid()` caches resources
+
+    In order to test the cache we much access the private class of
+    `json_schemas` and reset the LRU cache on `_LocalRefResolver`.
+    For this to happen, we need to disable errror `W0212`
+    on _LOCAL_RESOLVER
+    """
+    _LOCAL_RESOLVER._remote_cache.cache_clear()  # pylint: disable=W0212
+
+    assert_valid(make_empty_order(), "/orderSchema")
+    cache_info = (
+        _LOCAL_RESOLVER._remote_cache.cache_info()  # pylint: disable=W0212
+    )
+    assert cache_info.currsize == 4
+    assert cache_info.hits == 10