build: python 3 compatibility

Ubuntu 16.04 "xenial xerus" does not come with Python 2.x by default.
It is possible to install a python-2.7 package, but this has its own
problem: no python or python2 symlink (see #7717).

This fixes the following scripts to work with python 3:

• make check (bctest,py, bitcoin-util-test.py)
• make translate (extract_strings_qt.py)
• make symbols-check (symbol-check.py)
• make security-check (security-check.py)

Explicitly call the python commands using $(PYTHON) instead
of relying on the interpreter line at the top of the scripts.