Merge bitcoin/bitcoin#28211: Bump python minimum supported version to 3.9

fa25e8b0a1 doc: Recommend lint image build on every call (MarcoFalke)
faf70c1f33 Bump python minimum version to 3.9 (MarcoFalke)
fa8996b930 ci: Bump i686_multiprocess.sh to latest Ubuntu LTS (MarcoFalke)

Pull request description:

  All supported operating systems ship with python 3.9 (or later), so bumping the minimum should not cause any issues. A bump will allow new code to use new python 3.9 features.

  For reference:
  * https://packages.debian.org/bullseye/python3
  * https://packages.ubuntu.com/focal/python3.9
  * FreeBSD 12/13 also ships with 3.9
  * CentOS-like 8/9 also ships with 3.9 (and 3.11)
  * OpenSuse Leap also ships with 3.9 (and 3.11) https://software.opensuse.org/package/python311-base

  This is for Bitcoin Core 27.0 in 2024 (next year), not the soon upcoming 26.0 next month.

ACKs for top commit:
  Sjors:
    ACK fa25e8b0a1
  jamesob:
    ACK fa25e8b0a1 ([`jamesob/ackr/28211.1.MarcoFalke.bump_python_minimum_supp`](https://github.com/jamesob/bitcoin/tree/ackr/28211.1.MarcoFalke.bump_python_minimum_supp))

Tree-SHA512: 86c9f6ac4b5ba94a62ee6a6062dd48a8295d8611a39cdb5829f4f0dbc77aaa1a51edccc7a99275bf699143ad3a6fe826de426d413e5a465e3b0e82b86d10c32e

test/functional/test_framework/util.py

@@ -287,10 +287,10 @@ def sha256sum_file(filename):
     return h.digest()
 
 
-# TODO: Remove and use random.randbytes(n) instead, available in Python 3.9
+# TODO: Remove and use random.randbytes(n) directly
 def random_bytes(n):
     """Return a random bytes object of length n."""
-    return bytes(random.getrandbits(8) for i in range(n))
+    return random.randbytes(n)
 
 
 # RPC/P2P connection constants and functions