Switch to using fastjsonschema for schema validation. (#809)

The main reason to do this is because jsonschema (what
we currently use) depends on referencing, which depends
on rpds-py, which has C libraries.  That means that when
using the python debug interpreter on Windows, we would
have to build a custom pip package for it to work.

In contrast, fastjsonschema is pure python and has no
dependencies.  It is also available on all of our default
platforms.  So switch to using it here so things work on
Windows debug.

Signed-off-by: Chris Lalancette <clalancette@gmail.com>

rosidl_generator_tests/package.xml

@@ -24,7 +24,7 @@
   <test_depend>ament_lint_auto</test_depend>
   <test_depend>ament_lint_common</test_depend>
   <test_depend>ament_index_python</test_depend>
-  <test_depend>python3-jsonschema</test_depend>
+  <test_depend>python3-fastjsonschema</test_depend>
   <test_depend>rosidl_cmake</test_depend>
   <test_depend>rosidl_generator_c</test_depend>
   <test_depend>rosidl_generator_cpp</test_depend>

rosidl_generator_tests/test/rosidl_generator_type_description/test_type_hash.py

@@ -17,16 +17,18 @@
 from pathlib import Path
 
 from ament_index_python import get_package_share_directory
-import jsonschema
+import fastjsonschema
 
 
 def test_type_hash():
     """Test all rosidl_generator_type_description output files against defined schemas."""
     schema_path = (
         Path(get_package_share_directory('rosidl_generator_type_description')) / 'resource' /
         'HashedTypeDescription.schema.json')
+
     with schema_path.open('r') as schema_file:
         schema = json.load(schema_file)
+        validator = fastjsonschema.compile(schema)
 
     generated_files_dir = Path(os.environ['GENERATED_TEST_FILE_DIR'])
     validated_files = 0
@@ -36,6 +38,6 @@ def test_type_hash():
             assert p.suffix == '.json'
             with p.open('r') as f:
                 instance = json.load(f)
-            jsonschema.validate(instance=instance, schema=schema)
+            validator(instance)
             validated_files += 1
     assert validated_files, 'Needed to validate at least one JSON output.'