package/python3: security bump to version 3.9.7

package/python3: security bump to version 3.9.7

Fixes the following security issues:

- bpo-42278: Replaced usage of tempfile.mktemp() with TemporaryDirectory to
  avoid a potential race condition.

- bpo-41180: Add auditing events to the marshal module, and stop raising
  code.__init__ events for every unmarshalled code object.  Directly
  instantiated code objects will continue to raise an event, and audit event
  handlers should inspect or collect the raw marshal data.  This reduces a
  significant performance overhead when loading from .pyc files.

- bpo-44394: Update the vendored copy of libexpat to 2.4.1 (from 2.2.8) to
  get the fix for the CVE-2013-0340 “Billion Laughs” vulnerability.  This
  copy is most used on Windows and macOS.

- bpo-43124: Made the internal putcmd function in smtplib sanitize input for
  presence of \r and \n characters to avoid (unlikely) command injection.

https://www.python.org/downloads/release/python-397/

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>