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.

Makefile.am

@@ -110,7 +110,7 @@ osx_volname:
 
 if BUILD_DARWIN
 $(OSX_DMG): $(OSX_APP_BUILT) $(OSX_PACKAGING)
-	$(OSX_DEPLOY_SCRIPT) $(OSX_APP) -add-qt-tr $(OSX_QT_TRANSLATIONS) -translations-dir=$(QT_TRANSLATION_DIR) -dmg -fancy $(OSX_FANCY_PLIST) -verbose 2 -volname $(OSX_VOLNAME)
+	$(PYTHON) $(OSX_DEPLOY_SCRIPT) $(OSX_APP) -add-qt-tr $(OSX_QT_TRANSLATIONS) -translations-dir=$(QT_TRANSLATION_DIR) -dmg -fancy $(OSX_FANCY_PLIST) -verbose 2 -volname $(OSX_VOLNAME)
 
 deploydir: $(OSX_DMG)
 else
@@ -134,10 +134,10 @@ $(APP_DIST_DIR)/.background/$(OSX_BACKGROUND_IMAGE): $(OSX_BACKGROUND_IMAGE_DPIF
 	$(TIFFCP) -c none $(OSX_BACKGROUND_IMAGE_DPIFILES) $@
 
 $(APP_DIST_DIR)/.DS_Store: $(OSX_DSSTORE_GEN)
-	$< "$@" "$(OSX_VOLNAME)"
+	$(PYTHON) $< "$@" "$(OSX_VOLNAME)"
 
 $(APP_DIST_DIR)/$(OSX_APP)/Contents/MacOS/Bitcoin-Qt: $(OSX_APP_BUILT) $(OSX_PACKAGING)
-	INSTALLNAMETOOL=$(INSTALLNAMETOOL)  OTOOL=$(OTOOL) STRIP=$(STRIP) $(OSX_DEPLOY_SCRIPT) $(OSX_APP) -translations-dir=$(QT_TRANSLATION_DIR) -add-qt-tr $(OSX_QT_TRANSLATIONS) -verbose 2
+	INSTALLNAMETOOL=$(INSTALLNAMETOOL)  OTOOL=$(OTOOL) STRIP=$(STRIP) $(PYTHON) $(OSX_DEPLOY_SCRIPT) $(OSX_APP) -translations-dir=$(QT_TRANSLATION_DIR) -add-qt-tr $(OSX_QT_TRANSLATIONS) -verbose 2
 
 deploydir: $(APP_DIST_EXTRAS)
 endif