Use compat.string_types to check for visit name string
Use compat.string_types to check for visit name string

Th visitor optimization looking for plain string needs to
accommodate for u"" in Python 2 due to the unicode_literals
future.

Fixes: #4800
Change-Id: I5e3136f2f2cbd5f24d89186b599f59ea2e7f1550

j5int/sqlalchemy,wujuguang/sqlalchemy,monetate/sqlalchemy,sqlalchemy/sqlalchemy,zzzeek/sqlalchemy,graingert/sqlalchemy,graingert/sqlalchemy,j5int/sqlalchemy,monetate/sqlalchemy,wujuguang/sqlalchemy