Default to python3

Currently NTPsec's Python script look look for the python binary, which is Python2. As distributions are trying to get rid of Python2 dependencies, patching work is required, e.g.:

https://sources.debian.org/src/ntpsec/1.1.0+dfsg1-1/debian/patches/hardcode-python3-path.patch/

Python 3 is now 10 years old. I suggest to s/python/python3/ in the shebangs.