Add symlink to current appenv.

[sweh]

This allows adding a stable path to the Python interpreter in IDEs like VS Code.

[elikoga]

Once #38 is merged, I'd add this test:

def test_prepare_creates_venv_symlink(workdir, monkeypatch):
    # asserts that appenv_dir / "current" -> env_dir
    monkeypatch.setattr('sys.stdin', io.StringIO('ducker\nducker<2.0.2\n\n'))
    os.makedirs(os.path.join(workdir, 'ducker'))

    env = appenv.AppEnv(os.path.join(workdir, 'ducker'))
    env.init()
    env.update_lockfile()
    env.prepare()
    assert os.path.islink(os.path.join(env.appenv_dir, "current"))
    assert os.readlink(os.path.join(env.appenv_dir, "current")) == env.env_dir

to the test file introduced there. I could create the commit before, but it'd merge conflict when rebasing later

[zagy]

I'd actually prefer os.remove, but 🤷