pmbootstrap.py: Check which version of Python pmbootstrap is run with (MR 2118)

This prevents people from getting strange syntax errors if they
accidentally run pmbootstrap with e.g. Python 2.

pmbootstrap.py

@@ -1,8 +1,15 @@
 #!/usr/bin/env python3
+# -*- encoding: UTF-8 -*-
 # Copyright 2021 Oliver Smith
 # SPDX-License-Identifier: GPL-3.0-or-later
 # PYTHON_ARGCOMPLETE_OK
 import sys
+version = sys.version_info
+if version < (3, 6):
+    print("You need at least Python 3.6 to run pmbootstrap")
+    print("(You are running it with Python " + str(version.major) +
+          "." + str(version.minor) + ")")
+    sys.exit()
 import pmb
 
 if __name__ == "__main__":