Bug 41282: Downgrade to Python 3.9.

MozBug 1924022 introduced a dependency on the Python built-in SSL
module.
This caused an error in our Linux builds, because we run them in a very
old version of Debian that still uses OpenSSL 1.1.0, which is not
compatible with Python SSL module since Python 3.10.
The less intrusive way to resolve this is to downgrade to Python 3.9.x,
which is still supported by all our projects.

Also, switch to hashes to verify the Python source tarball, as the
Python Software Foundation often rotates keys, which reduces the
advantages of verifying the signature rather than the hash for us.

projects/python/config

 # vim: filetype=yaml sw=2
-version: 3.11.3
+version: 3.9.20
 filename: 'python-[% c("var/build_id") %].tar.[% c("compress_tar") %]'
 container:
   use_container: 1
@@ -24,9 +24,7 @@ input_files:
   - project: container-image
   - name: python
     URL: 'https://www.python.org/ftp/python/[% c("version") %]/Python-[% c("version") %].tar.xz'
-    gpg_keyring: python.gpg
-    sig_ext: asc
-    file_gpg_id: 1
+    sha256sum: 6b281279efd85294d2d6993e173983a57464c0133956fbbb5536ec9646beaf0c
   - name: '[% c("var/compiler") %]'
     project: '[% c("var/compiler") %]'
     enable: '[% c("var/linux") %]'