Fix/python bindings python requirements

• Add the requirement Python >= 3.6 for the Python bindings (#275). If the version is less, a hopefully informative message, friendly, but still attention-grabbing message is printed.
• Remove the check for the Python interpreter which was unreachable anyway due to a check further up
• Fixes #297 (closed)
• Unconditionally define add_python_targets for ease of use

This MR only concerns the Python bindings not Python support for possible other usages. This is why I didn't require the Python version via the find_package mechanism for now.