RFC/DEP: `NPYReader._filename_re` should be a template string

[neutrinoceros]

I noticed in #653 that the expected pattern for a filename was kinda duplicated. I think it could be avoided using Python 3.14's t-strings, and a suitable backport impl for older Pythons.

[neutrinoceros]

I couldn't find a third party backport with a drop-in replacement of the strings.templatelib.Template class. Maybe it'll change in the future.