Add twine and towncrier as dev dependencies (#11233)

We don't pin them as we execute them as commands, rather than use them
as libs.

changelog.d/11233.misc

+Add `twine` and `towncrier` as dev dependencies, as they're used by the release script.

setup.py

@@ -132,6 +132,9 @@ CONDITIONAL_REQUIREMENTS["dev"] = (
         "GitPython==3.1.14",
         "commonmark==0.9.1",
         "pygithub==1.55",
+        # The following are executed as commands by the release script.
+        "twine",
+        "towncrier",
     ]
 )