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.fish

@@ -0,0 +1,25 @@
+# This file should be sourced, not executed!
+#
+# This is a port of detect_python.sh. More information are provided there.
+
+set -x ESP_PYTHON python
+
+for p_cmd in python python3;
+    echo "Checking \"$p_cmd\" ..."
+
+    set res ($p_cmd -c "import sys; print(sys.version_info.major)")
+    if [ "$res" = "3" ]
+        set -x ESP_PYTHON $p_cmd
+        break
+    end
+end
+
+$ESP_PYTHON --version
+if [ $status -ne 0 ]
+    echo "\"$ESP_PYTHON\" is not installed! Please see the documentation for how to install it."
+    # The following exit skips the rest of this file but won't exit fish where the script was sourced. This is not a
+    # fatal issue.
+    exit 1
+end
+
+echo "\"$ESP_PYTHON\" has been detected"