CI/azure: replace python-impacket with python3-impacket

As of this month Azure DevOps uses Ubuntu 20.04 LTS which
no longer supports Python 2 and instead ships Python 3.

Closes #6678

.azure-pipelines.yml

@@ -76,7 +76,7 @@ stages:
           configure: --enable-debug --disable-shared --disable-threaded-resolver
           tests: -n -t --shallow=40 !FTP
     steps:
-    - script: sudo apt-get update && sudo apt-get install -y stunnel4 python-impacket libzstd-dev libbrotli-dev $(install)
+    - script: sudo apt-get update && sudo apt-get install -y stunnel4 python3-impacket libzstd-dev libbrotli-dev $(install)
       displayName: 'apt install'
 
     - script: ./buildconf && ./configure --enable-warnings --enable-werror $(configure)