Make json.loads of Python >=3.6 decode bytes by itself

Quoting https://docs.python.org/3/library/json.html#json.loads :
> Changed in version 3.6: s can now be of type bytes or bytearray.
> The input encoding should be UTF-8, UTF-16 or UTF-32.

tests/assertions.py

@@ -15,7 +15,7 @@ def assert_is_json_bytes(b: bytes):
     assert isinstance(b, bytes)
 
     try:
-        json.loads(b.decode("utf-8"))
+        json.loads(b)
     except Exception as error:
         raise AssertionError() from error