Freeze the regress version

Ideally this pin is temporary. Right now, `regress==0.4.5` works as
expected on the unicode pattern, and the latest regress version does
not.

For workable CI, pin the version.

pyproject.toml

@@ -21,7 +21,7 @@ dependencies = [
     'tomli>=2.0;python_version<"3.11"',
     "ruamel.yaml==0.18.6",
     "jsonschema>=4.18.0,<5.0",
-    "regress>=0.4.0",
+    "regress==0.4.5",
     "requests<3.0",
     "click>=8,<9",
 ]