remove simplejson

- remove encoding detection backport, json.loads supports it directly
- use str.translate instead of multiple str.replace

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 envlist =
     py{38,37,36,py3}
-    py38-{simplejson,devel,lowest}
+    py38-{devel,lowest}
     style
     docs
 skip_missing_interpreters = true
@@ -24,8 +24,6 @@ deps =
     devel: https://github.com/pallets/itsdangerous/archive/master.tar.gz
     devel: https://github.com/pallets/click/archive/master.tar.gz
 
-    simplejson: simplejson
-
 commands =
     pip install -q -e examples/tutorial[test]
     pip install -q -e examples/javascript[test]