qa: Use sys.executable when invoking other Python scripts

This change fixes tests on systems where `python3` is not available
in the `PATH`, causing the shebang `#!/usr/bin/env python3` to fail.

test/functional/rpc_signer.py

@@ -9,6 +9,7 @@ See also wallet_signer.py for tests that require wallet context.
 """
 import os
 import platform
+import sys
 
 from test_framework.test_framework import BitcoinTestFramework
 from test_framework.util import (
@@ -20,10 +21,7 @@ from test_framework.util import (
 class RPCSignerTest(BitcoinTestFramework):
     def mock_signer_path(self):
         path = os.path.join(os.path.dirname(os.path.realpath(__file__)), 'mocks', 'signer.py')
-        if platform.system() == "Windows":
-            return "py -3 " + path
-        else:
-            return path
+        return sys.executable + " " + path
 
     def set_test_params(self):
         self.num_nodes = 4