awscli2: fixup python 3.9 fallout

from the applied patch:

 Replace use of deprecated base64.encodestring()

Replace the uses of deprecated base64.encodestring() in favor
of botocore.compat.encodebytes().  This fixes incompatibility with
Python 3.9 where the former function has finally been removed.

pkgs/tools/admin/awscli2/default.nix

@@ -1,4 +1,4 @@
-{ lib, python3, groff, less, fetchFromGitHub }:
+{ lib, python3, groff, less, fetchFromGitHub, fetchpatch }:
 let
   py = python3.override {
     packageOverrides = self: super: {
@@ -33,6 +33,13 @@ with py.pkgs; buildPythonApplication rec {
     sha256 = "sha256-LU9Tqzdi8ULZ5y3FbfSXdrip4NcxFkXRCTpVGo05LcM=";
   };
 
+  patches = [
+    (fetchpatch {
+      url = "https://github.com/mgorny/aws-cli/commit/85361123d2fa12eaedf912c046ffe39aebdd2bad.patch";
+      sha256 = "sha256-1Rb+/CY7ze1/DbJ6TfqHF01cfI2vixZ1dT91bmHTg/A=";
+    })
+  ];
+
   postPatch = ''
     substituteInPlace setup.py \
       --replace "awscrt==0.11.13" "awscrt" \