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.qt.include

@@ -391,7 +391,7 @@ SECONDARY: $(QT_QM)
 
 qt/bitcoinstrings.cpp: $(libbitcoin_server_a_SOURCES) $(libbitcoin_wallet_a_SOURCES)
 	@test -n $(XGETTEXT) || echo "xgettext is required for updating translations"
-	$(AM_V_GEN) cd $(srcdir); XGETTEXT=$(XGETTEXT) PACKAGE_NAME="$(PACKAGE_NAME)" COPYRIGHT_HOLDERS="$(COPYRIGHT_HOLDERS)" COPYRIGHT_HOLDERS_SUBSTITUTION="$(COPYRIGHT_HOLDERS_SUBSTITUTION)" ../share/qt/extract_strings_qt.py $^
+	$(AM_V_GEN) cd $(srcdir); XGETTEXT=$(XGETTEXT) PACKAGE_NAME="$(PACKAGE_NAME)" COPYRIGHT_HOLDERS="$(COPYRIGHT_HOLDERS)" COPYRIGHT_HOLDERS_SUBSTITUTION="$(COPYRIGHT_HOLDERS_SUBSTITUTION)" $(PYTHON) ../share/qt/extract_strings_qt.py $^
 
 translate: qt/bitcoinstrings.cpp $(QT_FORMS_UI) $(QT_FORMS_UI) $(BITCOIN_QT_CPP) $(BITCOIN_QT_H) $(BITCOIN_MM)
 	@test -n $(LUPDATE) || echo "lupdate is required for updating translations"

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

src/test/bctest.py

@@ -1,7 +1,7 @@
 # Copyright 2014 BitPay, Inc.
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
-
+from __future__ import division,print_function,unicode_literals
 import subprocess
 import os
 import json

src/test/bitcoin-util-test.py

@@ -2,7 +2,7 @@
 # Copyright 2014 BitPay, Inc.
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
-
+from __future__ import division,print_function,unicode_literals
 import os
 import bctest
 import buildenv