Update PEP 8 to recommend against trailing whitespace.

pep-0008.txt

@@ -473,6 +473,12 @@ Avoid extraneous whitespace in the following situations:
 Other Recommendations
 ---------------------
 
+- Avoid trailing whitespace anywhere.  Because it's usually invisible,
+  it can be confusing: e.g. a backslash followed by a space and a
+  newline does not count as a line continuation marker.  Some editors
+  don't preserve it and many projects (like CPython itself) have
+  pre-commit hooks that reject it.
+
 - Always surround these binary operators with a single space on either
   side: assignment (``=``), augmented assignment (``+=``, ``-=``
   etc.), comparisons (``==``, ``<``, ``>``, ``!=``, ``<>``, ``<=``,