Rename the extra for non-GPL format support in the pyproject.toml

We're still waiting on the upstream fix for #962 which will
allow installing the extra with either form, but as-is, this is
technically not valid in a pyproject.toml (and is throwing some
schemastore errors) because keys here have to use hyphens, not
underscores.

See https://packaging.python.org/en/latest/specifications/declaring-project-metadata/#dependencies-optional-dependencies

pyproject.toml

@@ -52,7 +52,7 @@ format = [
   "uri_template",
   "webcolors>=1.11",
 ]
-format_nongpl = [
+format-nongpl = [
   "fqdn",
   "idna",
   "isoduration",