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.

vcr/filters.py

@@ -95,7 +95,7 @@ def replace_post_data_parameters(request, replacements):
                         new_body[k] = rv
             request.body = new_body
         elif request.headers.get("Content-Type") == "application/json":
-            json_data = json.loads(request.body.decode("utf-8"))
+            json_data = json.loads(request.body)
             for k, rv in replacements.items():
                 if k in json_data:
                     ov = json_data.pop(k)

vcr/matchers.py

@@ -73,11 +73,8 @@ def _header_checker(value, header="Content-Type"):
 
 
 def _transform_json(body):
-    # Request body is always a byte string, but json.loads() wants a text
-    # string. RFC 7159 says the default encoding is UTF-8 (although UTF-16
-    # and UTF-32 are also allowed: hmmmmm).
     if body:
-        return json.loads(body.decode("utf-8"))
+        return json.loads(body)
 
 
 _xml_header_checker = _header_checker("text/xml")