form field `clean` now force-stringifies value

src/yamlhttpforms/form.py

@@ -61,7 +61,8 @@ class FormField:
     def valid_option(self, option: str) -> bool:
         return self.options is None or option in self.options.keys() or option in self.options.values()
 
-    def clean(self, value: str) -> str:
+    def clean(self, value: Any) -> str:
+        value = str(value)
         if self.options is None or value in self.options.keys():
             return value
         # Try to find an unambiguous match in the visible option texts: