tests: remove reliance on python3 name

* run key generation using current python interpreter
* remove explicit fail when python3 is not found

test/conftest.py

@@ -3,6 +3,7 @@
 import json
 import pathlib
 import subprocess
+import sys
 import time
 
 import pytest
@@ -18,6 +19,7 @@ def private_key(tmp_path_factory, tool_path):
         ['unsafe', 'example.com'],
     ]:
         binargs = [
+            sys.executable,
             tool_path('contrib/openarc-keygen'),
             '-D',
             str(basepath),

test/test_keygen.py

@@ -35,7 +35,6 @@ def test_keygen(tool_path, tmp_path, python_version):
     try:
         subprocess.run(binargs, check=True)
     except FileNotFoundError:
-        assert python_version != 'python3'
         pytest.skip(f'{python_version} not found')
 
     keystat = tmp_path.joinpath('foo._domainkey.example.com.key').stat()