fix: use sys.executable instead of hardcoded "python" in tests

Tests that launch the API server via subprocess used a hardcoded
"python" command which fails on systems where only "python3" is
available (e.g. macOS). Using sys.executable ensures the same
interpreter running pytest is used for subprocesses.

Fixes 36 test errors on macOS environments.

atroposlib/tests/test_api_compression.py

@@ -5,6 +5,7 @@ import json
 import os
 import signal
 import subprocess
+import sys
 import time
 
 import pytest
@@ -27,7 +28,7 @@ def wait_for_api_server(max_wait=10):
 def api_server():
     proc = subprocess.Popen(
         [
-            "python",
+            sys.executable,
             "-m",
             "atroposlib.cli.run_api",
             "--host",