Fix commas and whitespace in PEP 473.

pep-0473.txt

@@ -14,7 +14,7 @@ Abstract
 ========
 
 Exceptions like ``AttributeError``, ``IndexError``, ``KeyError``,
-``LookupError``, ``NameError``, , ``TypeError`` and ``ValueError`` do not
+``LookupError``, ``NameError``, ``TypeError``, and ``ValueError`` do not
 provide all information required by programmers to debug and better understand
 what caused them.
 Furthermore, in some cases the messages even have slightly different formats,
@@ -47,7 +47,7 @@ extremely fragile and not portable.
 A similar proposal [2]_ has been implemented for ``ImportError`` and in the same
 fashion this idea has received support [3]_. Additionally, almost 10 years ago
 Guido asked in [11]_ to have a clean API to access the affected objects in
-Exceptions like ``KeyError``, ``AttributeError``,  ``NameError`` and
+Exceptions like ``KeyError``, ``AttributeError``, ``NameError``, and
 ``IndexError``. Similar issues and proposals ideas have been written in the
 last year. Some other issues have been created, but despite receiving support
 they finally get abandoned. References to the created issues are listed below:
@@ -157,7 +157,7 @@ Proposal
 ========
 
 Extend the exceptions ``AttributeError``, ``IndexError``, ``KeyError``,
-``LookupError``, ``NameError``, , ``TypeError`` and ``ValueError`` with the
+``LookupError``, ``NameError``, ``TypeError``, and ``ValueError`` with the
 following:
 
 * ``AttributeError``: target :sup:`w`, attribute