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.

src/Makefile.test.include

@@ -129,7 +129,7 @@ bitcoin_test_clean : FORCE
 
 check-local:
 	@echo "Running test/bitcoin-util-test.py..."
-	$(AM_V_at)srcdir=$(srcdir) PYTHONPATH=$(builddir)/test $(srcdir)/test/bitcoin-util-test.py
+	$(AM_V_at)srcdir=$(srcdir) PYTHONPATH=$(builddir)/test $(PYTHON) $(srcdir)/test/bitcoin-util-test.py
 	$(AM_V_at)$(MAKE) $(AM_MAKEFLAGS) -C secp256k1 check
 if EMBEDDED_UNIVALUE
 	$(AM_V_at)$(MAKE) $(AM_MAKEFLAGS) -C univalue check