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/functional/feature_framework_startup_failures.py

@@ -43,15 +43,13 @@ class FeatureFrameworkStartupFailures(BitcoinTestFramework):
         args = [sys.executable] + sys.argv + [f"--tmpdir={self.options.tmpdir}/{subdir}", f"--internal_test={test.__name__}"] + internal_args
 
         try:
-            # The subprocess encoding argument gives different results for e.output
-            # on Linux/Windows, so handle decoding by ourselves for consistency.
             output = subprocess.run(args, timeout=60 * self.options.timeout_factor,
-                                    stdout=subprocess.PIPE, stderr=subprocess.STDOUT).stdout.decode("utf-8")
+                                    stdout=subprocess.PIPE, stderr=subprocess.STDOUT, text=True).stdout
         except subprocess.TimeoutExpired as e:
             print("Unexpected child process timeout!\n"
                   "WARNING: Timeouts like this halt execution of TestNode logic, "
                   "meaning dangling bitcoind processes are to be expected.\n"
-                  f"<CHILD OUTPUT BEGIN>\n{e.output.decode('utf-8')}\n<CHILD OUTPUT END>",
+                  f"<CHILD OUTPUT BEGIN>\n{e.output}\n<CHILD OUTPUT END>",
                   file=sys.stderr)
             raise
 

test/functional/tool_signet_miner.py

@@ -1,5 +1,5 @@
 #!/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.
 """Test signet miner tool"""
@@ -102,8 +102,8 @@ class SignetMinerTest(BitcoinTestFramework):
                 'genpsbt',
                 f'--address={node.getnewaddress()}',
                 '--poolnum=98',
-            ], check=True, input=json.dumps(template).encode('utf8'), capture_output=True)
-        psbt = genpsbt.stdout.decode('utf8').strip()
+            ], check=True, text=True, input=json.dumps(template), capture_output=True)
+        psbt = genpsbt.stdout.strip()
         if sign:
             self.log.debug("Sign the PSBT")
             res = node.walletprocesspsbt(psbt=psbt, sign=True, sighashtype='ALL')
@@ -112,8 +112,8 @@ class SignetMinerTest(BitcoinTestFramework):
         solvepsbt = subprocess.run(base_cmd + [
                 'solvepsbt',
                 f'--grind-cmd={shlex.join(util_argv)}',
-            ], check=True, input=psbt.encode('utf8'), capture_output=True)
-        node.submitblock(solvepsbt.stdout.decode('utf8').strip())
+            ], check=True, text=True, input=psbt, capture_output=True)
+        node.submitblock(solvepsbt.stdout.strip())
         assert_equal(node.getblockcount(), n_blocks + 1)
 
     def run_test(self):