Better note that str.zfill() was added in 2.2.2 as well. ;-(

Doc/lib/libstdtypes.tex

@@ -723,6 +723,7 @@ Return a copy of the string converted to uppercase.
 Return the numeric string left filled with zeros in a string
 of length \var{width}. The original string is returned if
 \var{width} is less than \code{len(\var{s})}.
+\versionadded{2.2.2}
 \end{methoddesc}