tools: Prefer python3 during install and export

Install and export script should work on systems without "python"
executable.

Closes https://github.com/espressif/esp-idf/pull/6471

Closes https://github.com/espressif/esp-idf/issues/6532

Related to https://github.com/espressif/esp-idf/issues/6421 and
https://github.com/espressif/arduino-esp32/issues/4717

tools/detect_python.sh

@@ -0,0 +1,24 @@
+# This file should be sourced, not executed!
+#
+# This is a helper script for detecting Python executables in the PATH. It is intended to be used for determining
+# which Python should be used with idf_tools.py for installing tools and exporting environment variables.
+#
+# 1. The script will set variable ESP_PYTHON to "python" if it is of version 3.
+# 2. Otherwise, "python3" will be exported if it exists.
+# 3. The script will fall-back to "python" as the last resort and fail if it doesn't exist.
+
+ESP_PYTHON=python
+
+for p_cmd in python python3
+do
+    echo "Checking \"$p_cmd\" ..."
+
+    if [ "$($p_cmd -c "import sys; print(sys.version_info.major)")" = 3 ]; then
+        ESP_PYTHON=$p_cmd
+        break
+    fi
+done
+
+$ESP_PYTHON --version || { echo "\"$ESP_PYTHON\" is not installed! Please see the documentation for how to install it."; exit 1; }
+
+echo "\"$ESP_PYTHON\" has been detected"