dep: Add cached-property package (for py37 or older)

CHANGES

@@ -4,6 +4,7 @@ Release 3.0.0 (in development)
 Dependencies
 ------------
 
+* Add ``cached-property`` package (for py37 or older)
 * LaTeX: drop dependency on :program:`extractbb` for image inclusion in
   Japanese documents as ``.xbb`` files are unneeded by :program:`dvipdfmx`
   since TeXLive2015 (refs: #6189)

setup.py

@@ -31,13 +31,17 @@
     'requests>=2.5.0',
     'setuptools',
     'packaging',
+    'cached-property',
 ]
 
 extras_require = {
     # Environment Marker works for wheel 0.24 or later
     ':sys_platform=="win32"': [
         'colorama>=0.3.5',
     ],
+    ':python_version<"3.8"': [
+        'cached-property',
+    ],
     'docs': [
         'sphinxcontrib-websupport',
     ],

sphinx/pycode/function.py

@@ -9,11 +9,17 @@
 """
 
 import ast
+import sys
 from collections import OrderedDict
 from inspect import Parameter
 from typing import cast
 from typing import Mapping
 
+if sys.version_info > (3, 8):
+    from functools import cached_property
+else:
+    from cached_property import cached_property
+
 
 def stringify(node: ast.AST) -> str:
     """Stringify an AST node."""
@@ -74,7 +80,7 @@ def __init__(self, signature: str) -> None:
         module = ast.parse('def func' + signature + ': pass')
         self.definition = cast(ast.FunctionDef, module.body[0])
 
-    @property
+    @cached_property
     def parameters(self) -> Mapping:
         args = self.definition.args
         params = []
@@ -113,6 +119,6 @@ def parameters(self) -> Mapping:
 
         return OrderedDict((p.name, p) for p in params)
 
-    @property
+    @cached_property
     def return_annotation(self) -> str:
         return stringify(self.definition.returns)