make Pyre happy

Summary:
Less red files in VS Code

Code regenerated by
```
fbcode (aa3405973)]$ buck2 run //glean/schema/gen:gen-schema -- --py glean/schema/ --dir glean/schema/source
```

Reviewed By: josefs

Differential Revision: D55923141

fbshipit-source-id: eea64c5f7fe94408e05e6b57da96a489d1ee7358

glean/schema/gen/Glean/Schema/Gen/Python.hs

@@ -58,8 +58,8 @@ genSchemaPy _version preddefs typedefs =
   , "T = TypeVar(\"T\")"
   , ""
   , "class Just(Generic[T]):"
-  , "  " <> "just: T = None"
-  , "  " <> "def __init__(self, just: T = None) -> None:"
+  , "  " <> "just: T | None = None"
+  , "  " <> "def __init__(self, just: T | None = None) -> None:"
   , "    " <> "self.just = just"
   , "  " <> "def get(self) -> Optional[T]:"
   , "    " <> "return self.just"