[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
@@ -364,7 +366,7 @@ module-level scope:
     IMO the inability of referencing class-level definitions
     from annotations on methods pretty much kills this idea.
 
-    https://mail.python.org/pipermail/python-dev/2017-November/150109.html
+https://mail.python.org/pipermail/python-dev/2017-November/150109.html
 
 This led to a short discussion about extending lambda-ized
 annotations for methods to be able to refer to class-level
@@ -373,7 +375,7 @@ scope.  This idea, too, was quickly rejected.
 
 PEP 563 summarizes the above discussion here:
 
-    https://www.python.org/dev/peps/pep-0563/#keeping-the-ability-to-use-function-local-state-when-defining-annotations
+https://www.python.org/dev/peps/pep-0563/#keeping-the-ability-to-use-function-local-state-when-defining-annotations
 
 What's puzzling is PEP 563's own changes to the scoping rules
 of annotations—it *also* doesn't permit annotations to reference
@@ -400,7 +402,7 @@ Implementation
 
 There's a prototype implementation of this PEP, here:
 
-    https://github.com/larryhastings/co_annotations/
+https://github.com/larryhastings/co_annotations/
 
 As of this writing, all features described in this PEP are
 implemented, and there are some rudimentary tests in the