Cross-reference PEP 3131 from PEP 8 (#286)

PEP 3131 (Unicode identifiers) includes a policy section
covering their use within the standard library.

This adds an explicit cross-reference to that section from
the prescriptive naming conventions section of PEP 8.

pep-0008.txt

@@ -850,6 +850,14 @@ names.
 In some fonts, these characters are indistinguishable from the
 numerals one and zero.  When tempted to use 'l', use 'L' instead.
 
+ASCII Compatibility
+~~~~~~~~~~~~~~~~~~~
+
+Identifiers used in the standard library must be ASCII compatible
+as described in the
+`policy section <https://www.python.org/dev/peps/pep-3131/#policy-specification>`_
+of PEP 3131.
+
 Package and Module Names
 ~~~~~~~~~~~~~~~~~~~~~~~~