tests: do not run the deprecated python3 tests unless python is python3

It's broken in e.g. pypy, but that's okay because the module is utterly
deprecated, and people should use the python module instead.

run_project_tests.py

@@ -1104,7 +1104,7 @@ def detect_tests_to_run(only: T.Dict[str, T.List[str]], use_tmp: bool) -> T.List
         TestCategory('swift', 'swift', backend not in (Backend.ninja, Backend.xcode) or not shutil.which('swiftc')),
         # CUDA tests on Windows: use Ninja backend:  python run_project_tests.py --only cuda --backend ninja
         TestCategory('cuda', 'cuda', backend not in (Backend.ninja, Backend.xcode) or not shutil.which('nvcc')),
-        TestCategory('python3', 'python3', backend is not Backend.ninja),
+        TestCategory('python3', 'python3', backend is not Backend.ninja or 'python3' not in sys.executable),
         TestCategory('python', 'python'),
         TestCategory('fpga', 'fpga', shutil.which('yosys') is None),
         TestCategory('frameworks', 'frameworks'),
@@ -1591,7 +1591,7 @@ if __name__ == '__main__':
     clear_transitive_files()
 
     print('Meson build system', meson_version, 'Project Tests')
-    print('Using python', sys.version.split('\n')[0])
+    print('Using python', sys.version.split('\n')[0], f'({sys.executable!r})')
     if 'VSCMD_VER' in os.environ:
         print('VSCMD version', os.environ['VSCMD_VER'])
     setup_commands(options.backend)