LatinRules.xdy now encoded as UTF-8

Previously, sphinx/texinputs/LatinRules.xdy was encoded as
(as reported by `file LatinRules.xdy`):

  > Non-ISO extended-ASCII text, with LF, NEL line terminators

This has at least two consequences:
1. More basic text readers display the file incorrectly. This includes
   the file's own [raw view](https://raw.githubusercontent.com/sphinx-doc/sphinx/31eba1a76dd485dc633cae48227b46879eda5df4/sphinx/texinputs/LatinRules.xdy) on GitHub.
2. Certain pipelines may automatically convert this to UTF-8 (ostensibly
   a good thing), which causes headaches with diff and version control
   (500 spurious line changes every time the document is rebuilt).