Remove "Release signatures" section of the README

PyPI has stopped supporting signed releases.

https://blog.pypi.org/posts/2023-05-23-removing-pgp/

Author: AA-Turner

README.rst

@@ -58,15 +58,6 @@ Contributing
 We appreciate all contributions! Refer to `the contributors guide`_ for
 information.
 
-Release signatures
-==================
-
-Releases are signed with following keys:
-
-* `498D6B9E <https://pgp.mit.edu/pks/lookup?op=vindex&search=0x102C2C17498D6B9E>`_
-* `5EBA0E07 <https://pgp.mit.edu/pks/lookup?op=vindex&search=0x1425F8CE5EBA0E07>`_
-* `61F0FB52 <https://pgp.mit.edu/pks/lookup?op=vindex&search=0x52C8F72A61F0FB52>`_
-
 .. _the documentation: https://www.sphinx-doc.org/
 .. _the contributors guide: https://www.sphinx-doc.org/en/master/internals/contributing.html
 .. _Python Package Index: https://pypi.org/project/Sphinx/

utils/release-checklist

@@ -35,7 +35,7 @@ Build Sphinx
 
 * ``make clean``
 * ``python -m build .``
-* ``twine upload dist/Sphinx-* --sign --identity [your GPG key]``
+* ``twine upload dist/Sphinx-*``
 * open https://pypi.org/project/Sphinx/ and check for any obvious errors
 
 for stable and major releases