textproc/py-fluent.syntax: Add py-fluent.syntax 0.18.1

Project Fluent is a collection of Python packages to use the Fluent localization
system.

The syntax package includes the parser, serializer, and traversal utilities like
Visitor and Transformer. You're looking for this package if you work on tooling
for Fluent in Python.

WWW: https://github.com/projectfluent/python-fluent
WWW: https://github.com/projectfluent/python-fluent/tree/master/fluent.syntax