diff --git a/sphinx/util/stemmer/__init__.py b/sphinx/util/stemmer/__init__.py
index 047aac708..bda5d2bc2 100644
--- a/sphinx/util/stemmer/__init__.py
+++ b/sphinx/util/stemmer/__init__.py
@@ -18,18 +18,15 @@ except ImportError:
 
 
 class BaseStemmer:
-    def stem(self, word):
-        # type: (str) -> str
+    def stem(self, word: str) -> str:
         raise NotImplementedError()
 
 
 class PyStemmer(BaseStemmer):
-    def __init__(self):
-        # type: () -> None
+    def __init__(self) -> None:
         self.stemmer = _PyStemmer('porter')
 
-    def stem(self, word):
-        # type: (str) -> str
+    def stem(self, word: str) -> str:
         return self.stemmer.stemWord(word)
 
 
@@ -37,13 +34,11 @@ class StandardStemmer(PorterStemmer, BaseStemmer):  # type: ignore
     """All those porter stemmer implementations look hideous;
     make at least the stem method nicer.
     """
-    def stem(self, word):  # type: ignore
-        # type: (str) -> str
+    def stem(self, word: str) -> str:  # type: ignore
         return super().stem(word, 0, len(word) - 1)
 
 
-def get_stemmer():
-    # type: () -> BaseStemmer
+def get_stemmer() -> BaseStemmer:
     if PYSTEMMER:
         return PyStemmer()
     else: