gh-142999: Make test_venv use sysconfig._get_implementation().lower() instead of hardcoding "python"

[youknowone]

instead of hard-coded "python"

I tried to use EnvBuilder._venv_path or sysconfig.get_path('purelib', scheme='venv'), but that complicates IMPL_NAME initialization because they returns abstract path. Please tell me if that's better.

• Issue: Implementation name "python" is hard-coded in test_venv #142999