Fix the bump_version script's changelog mutation

HACKING.md

@@ -136,7 +136,8 @@ To release snooty, do the following:
   in the `dist` directory, and check the _This is a pre-release_ checkbox.
 
 If there is an error, use `git reset --hard <previous_commit_hash>` to revert any
-commits that might have been made.
+commits that might have been made, and `git tag --delete v<version>` to remove the
+tag if it was created.
 
 ## Problem Areas
 

tools/bump_version.py

@@ -44,7 +44,7 @@ def release_changelog(version: str, text: str) -> str:
 
     def replace(match: Match[str]) -> str:
         unreleased_block = match["unreleased"].strip()
-        return f"\n## [Unreleased]\n\n## [{version}] - {date_string}\n\n{unreleased_block}\n"
+        return f"\n## [Unreleased]\n\n## [v{version}] - {date_string}\n\n{unreleased_block}\n"
 
     result = CHANGELOG_UNRELEASED_PAT.sub(replace, text)
     if not result:

tools/test_bump_version.py

@@ -31,7 +31,7 @@ def test_release_changelog() -> None:
 - No longer create spurious diagnostics about including apiargs artifacts and `hash.rst`."""
 
     assert (
-        release_changelog("v0.1.13", TEST_INPUT1)
+        release_changelog("0.1.13", TEST_INPUT1)
         == f"""# Changelog
 
 ## [Unreleased]