tests/*.py: support Python 2 again while preferring Python 3

This makes it possible to use the Python-based test servers
again in legacy environments which do not have modern Python.

Install Python package impacket in relevant CI workflows.

Follow up to curl#7602 which broke Python 2 support
Follow up to curl#7935 which fixed impacket support

.github/workflows/linux-hyper.yml

@@ -28,6 +28,9 @@ jobs:
     - run: sudo apt-get install libtool autoconf automake pkg-config
       name: install prereqs
 
+    - run: python3 -m pip install impacket
+      name: 'pip3 install'
+
     - run: (cd $HOME;
         git clone --depth=1 https://github.com/hyperium/hyper.git;
         curl https://sh.rustup.rs -sSf | sh -s -- -y;

.github/workflows/macos.yml

@@ -76,6 +76,9 @@ jobs:
     - run: brew update && for i in 1 2 3; do brew bundle install --no-lock --file /tmp/Brewfile && break || sleep 1; done
       name: 'brew install'
 
+    - run: python3 -m pip install impacket
+      name: 'pip3 install'
+
     - uses: actions/checkout@v2
 
     - run: ./buildconf && ./configure --enable-warnings --enable-werror ${{ matrix.build.configure }}
@@ -123,6 +126,9 @@ jobs:
     - run: brew update && brew bundle install --no-lock --file /tmp/Brewfile
       name: 'brew install'
 
+    - run: python3 -m pip install impacket
+      name: 'pip3 install'
+
     - uses: actions/checkout@v2
 
     - run: cmake -H. -Bbuild -DCURL_WERROR=ON -DPICKY_COMPILER=ON ${{ matrix.build.generate }}

tests/dictserver.py

@@ -1,6 +1,5 @@
-#!/usr/bin/env python3
+#!/bin/sh
 # -*- coding: utf-8 -*-
-#***************************************************************************
 #                                  _   _ ____  _
 #  Project                     ___| | | |  _ \| |
 #                             / __| | | | |_) | |
@@ -20,13 +19,17 @@
 # This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 # KIND, either express or implied.
 #
-###########################################################################
 #
-""" DICT server """
-
+# The following 3 lines implement a fallback from the interpreter named python3
+# to the interpreter named python. This way we prefer Python 3 over Python 2.
+# It works by executing the 2nd line using a shell which then executes Python.
+""":"
+exec "$(command -v python3 || command -v python)" "$0" "$@"
+":"""
 from __future__ import (absolute_import, division, print_function,
                         unicode_literals)
 
+""" DICT server"""
 import argparse
 import logging
 import os

tests/negtelnetserver.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/bin/sh
 # -*- coding: utf-8 -*-
 #
 #  Project                     ___| | | |  _ \| |
@@ -19,11 +19,17 @@
 # This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 # KIND, either express or implied.
 #
-""" A telnet server which negotiates"""
-
+#
+# The following 3 lines implement a fallback from the interpreter named python3
+# to the interpreter named python. This way we prefer Python 3 over Python 2.
+# It works by executing the 2nd line using a shell which then executes Python.
+""":"
+exec "$(command -v python3 || command -v python)" "$0" "$@"
+":"""
 from __future__ import (absolute_import, division, print_function,
                         unicode_literals)
 
+""" A telnet server which negotiates"""
 import argparse
 import logging
 import os

tests/smbserver.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/bin/sh
 # -*- coding: utf-8 -*-
 #
 #  Project                     ___| | | |  _ \| |
@@ -19,11 +19,17 @@
 # This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 # KIND, either express or implied.
 #
-"""Server for testing SMB"""
-
+#
+# The following 3 lines implement a fallback from the interpreter named python3
+# to the interpreter named python. This way we prefer Python 3 over Python 2.
+# It works by executing the 2nd line using a shell which then executes Python.
+""":"
+exec "$(command -v python3 || command -v python)" "$0" "$@"
+":"""
 from __future__ import (absolute_import, division, print_function,
                         unicode_literals)
 
+"""Server for testing SMB"""
 import argparse
 import logging
 import os

tests/util.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python3
+#!/bin/sh
 # -*- coding: utf-8 -*-
 #
 #  Project                     ___| | | |  _ \| |
@@ -19,11 +19,17 @@
 # This software is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY
 # KIND, either express or implied.
 #
-"""Module for extracting test data from the test data folder and other utils"""
-
+#
+# The following 3 lines implement a fallback from the interpreter named python3
+# to the interpreter named python. This way we prefer Python 3 over Python 2.
+# It works by executing the 2nd line using a shell which then executes Python.
+""":"
+exec "$(command -v python3 || command -v python)" "$0" "$@"
+":"""
 from __future__ import (absolute_import, division, print_function,
                         unicode_literals)
 
+"""Module for extracting test data from the test data folder and other utils"""
 import logging
 import os
 import re