Fix schema generation error caused by new click version (#4750)

* Fix schema generation error caused by new click version
Not sure on the policy for updating versions, so to cope with the new click 8.3.0 version I used some code from #4577 for version conditional behavior. When >= 8.3.0, unset defaults are now the special UNSET sentinal instead of None.
Hopefully nothing breaks on the click._utils usage since it's an _ module, might have to find a different solution if it does.

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Author: MeGaGiGaGon

scripts/generate_schema.py

@@ -1,7 +1,9 @@
 import json
+from importlib.metadata import version as imp_version
 from typing import IO, Any
 
 import click
+from packaging.version import Version
 
 import black
 
@@ -38,6 +40,11 @@ def generate_schema_from_click(
         result[name]["description"] = param.help
 
         if param.default is not None and not param.multiple:
+            if Version(imp_version("click")) >= Version("8.3.0"):
+                # Ignore the attr-defined error from running with click < 8.3.0, and
+                # also ignore the error for unused-ignore with click >= 8.3.0
+                if param.default is click._utils.UNSET:  # type: ignore[attr-defined, unused-ignore]
+                    continue
             result[name]["default"] = param.default
 
     return result