Use sys.executable rather than "python" to invoke pytest

This supports environments where the Python interpreter is not named
"python". For example, in Fedora it is "python3" unless the
python-unversioned-command package is installed.

src/inline_snapshot/testing/_example.py

@@ -5,6 +5,7 @@
 import platform
 import re
 import subprocess as sp
+import sys
 import traceback
 from argparse import ArgumentParser
 from io import StringIO
@@ -263,7 +264,7 @@ def run_pytest(
             tmp_path = Path(dir)
             self._write_files(tmp_path)
 
-            cmd = ["python", "-m", "pytest", *args]
+            cmd = [sys.executable, "-m", "pytest", *args]
 
             term_columns = 80