Add cookbook recipe for non-ascii migration files under python 2
Add cookbook recipe for non-ascii migration files under python 2

Unicode-related directives can be added to ``script.py.mako``
and as this issue has never been reported for many years as well
as that Python 2 is deprecated, resolve this as a cookbook recipe
that illustrates where unicode related directives and conversions
need to occur under Python 2.

Change-Id: Ib8b5446c0ef26db8a99f2ca26da000e9134102e9
Fixes: #584

zzzeek/alembic,sqlalchemy/alembic