Add typing for context managers returning Self

`Self` was introduced in python 3.11. Use `Self` from typing_extensions for previous Python versions

sphinxtools/utilities.py

@@ -32,6 +32,10 @@ from .constants import CPP_ITEMS, VERSION, VALUE_MAP
 from .constants import RE_KEEP_SPACES, EXTERN_INHERITANCE
 from .constants import DOXYROOT, SPHINXROOT, WIDGETS_IMAGES_ROOT
 
+if sys.version_info >= (3, 11):
+    from typing import Self
+else:
+    from typing_extensions import Self
 
 # ----------------------------------------------------------------------- #
 
@@ -622,7 +626,7 @@ class PickleFile(object):
     def __init__(self, fileName):
         self.fileName = fileName
 
-    def __enter__(self):
+    def __enter__(self) -> Self:
         self.read()
         return self