bpo-39435: Make the first argument of pickle.loads() positional-only. (GH-19846)

It was positional-only de facto: documentation and two implementations
used three different name.
diff --git a/Doc/library/pickle.rst b/Doc/library/pickle.rst
index d92e947..b7c3452 100644
--- a/Doc/library/pickle.rst
+++ b/Doc/library/pickle.rst
@@ -252,7 +252,7 @@
    .. versionchanged:: 3.8
       The *buffers* argument was added.
 
-.. function:: loads(data, \*, fix_imports=True, encoding="ASCII", errors="strict", buffers=None)
+.. function:: loads(data, /, \*, fix_imports=True, encoding="ASCII", errors="strict", buffers=None)
 
    Return the reconstituted object hierarchy of the pickled representation
    *data* of an object. *data* must be a :term:`bytes-like object`.