Python 3.9.21 security release.

Security content in this release

• gh-122792: Changed IPv4-mapped ipaddress.IPv6Address to consistently use the mapped IPv4 address value for deciding properties. Properties which have their behavior fixed are is_multicast, is_reserved, is_link_local, is_global, and is_unspecified.
• CVE-2024-9287: gh-124651: Properly quote template strings in venv activation scripts.
• gh-103848: Added checks to ensure that [ bracketed ] hosts found by urllib.parse.urlsplit() are of IPv6 or IPvFuture format.
• gh-95588: Clarified the conflicting advice given in the ast documentation about ast.literal_eval() being "safe" for use on untrusted input while at the same time warning that it can crash the process. The latter statement is true and is deemed unfixable without a large amount of work unsuitable for a bugfix. So we keep the warning and no longer claim that literal_eval is safe.