contrib: Use text=True in subprocess over manual encoding handling

All touched Python scripts already assume and require UTF8, so manually
specifying encoding or decoding for functions in the subprocess module
is redundant to just using text=True, which exists since Python 3.7

test/lint/lint-python-dead-code.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python3
 #
-# Copyright (c) 2022 The Bitcoin Core developers
+# Copyright (c) 2022-present The Bitcoin Core developers
 # Distributed under the MIT software license, see the accompanying
 # file COPYING or http://www.opensource.org/licenses/mit-license.php.
 
@@ -24,15 +24,15 @@ def check_vulture_install():
 def main():
     check_vulture_install()
 
-    files = check_output(FILES_ARGS).decode("utf-8").splitlines()
+    files = check_output(FILES_ARGS, text=True).splitlines()
     # --min-confidence 100 will only report code that is guaranteed to be unused within the analyzed files.
     # Any value below 100 introduces the risk of false positives, which would create an unacceptable maintenance burden.
     vulture_args = ['vulture', '--min-confidence=100'] + files
 
     try:
-        check_output(vulture_args, stderr=STDOUT)
+        check_output(vulture_args, stderr=STDOUT, text=True)
     except CalledProcessError as e:
-        print(e.output.decode("utf-8"), end="")
+        print(e.output, end="")
         print("Python dead code detection found some issues")
         exit(1)