fix FilePath and DirectoryPath (#226)

pydantic/types.py

@@ -329,6 +329,7 @@ class FilePath(Path):
         yield path_exists_validator
         yield cls.validate
 
+    @classmethod
     def validate(cls, value: Path) -> Path:
         if not value.is_file():
             raise errors.PathNotAFileError(path=value)
@@ -343,6 +344,7 @@ class DirectoryPath(Path):
         yield path_exists_validator
         yield cls.validate
 
+    @classmethod
     def validate(cls, value: Path) -> Path:
         if not value.is_dir():
             raise errors.PathNotADirectoryError(path=value)