scripted-diff: Specify Python major version explicitly on Windows

Using `py.exe` launcher might by fragile depending on how Python was
installed. Specifying the Python version explicitly fixes test errors
like this:
```
RunCommandParseJSON error: process(py C:\Users\hebasto\bitcoin\test\functional\mocks\signer.py enumerate) returned 9009: Python was not found...
```

-BEGIN VERIFY SCRIPT-
sed -i 's|"py "|"py -3 "|g' $(git grep -l '"py "' -- test/functional)
-END VERIFY SCRIPT-

Author: hebasto

test/functional/rpc_signer.py

@@ -21,7 +21,7 @@ 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 " + path
+            return "py -3 " + path
         else:
             return path
 

test/functional/wallet_signer.py

@@ -25,21 +25,21 @@ def add_options(self, parser):
     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 " + path
+            return "py -3 " + path
         else:
             return path
 
     def mock_invalid_signer_path(self):
         path = os.path.join(os.path.dirname(os.path.realpath(__file__)), 'mocks', 'invalid_signer.py')
         if platform.system() == "Windows":
-            return "py " + path
+            return "py -3 " + path
         else:
             return path
 
     def mock_multi_signers_path(self):
         path = os.path.join(os.path.dirname(os.path.realpath(__file__)), 'mocks', 'multi_signers.py')
         if platform.system() == "Windows":
-            return "py " + path
+            return "py -3 " + path
         else:
             return path