[pep-649] Clean up formatting (GH-2048)

One code-block was added, and fixed a couple of places where URLs were formatted as quotations.

pep-0649.rst

@@ -326,6 +326,8 @@ from within the language.  Therefore it's possible to write code
 that behaves differently based on whether annotations are
 evaluated at binding time or at access time, e.g.
 
+.. code-block::
+
     mytype = str
     def foo(a:mytype): pass
     mytype = int