Fix --outdir to pass a str to webbrowser.open_new_tab()

[hugovk]

❯ python --version
Python 3.12.4
❯ python -X importtime -c 'import pprint' 2> import.log && tuna import.log -o /tmp/tuna-html
Exception in thread Thread-1 (<lambda>):
Traceback (most recent call last):
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/threading.py", line 1073, in _bootstrap_inner
    self.run()
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/threading.py", line 1010, in run
    self._target(*self._args, **self._kwargs)
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/site-packages/tuna/cli.py", line 29, in <lambda>
    target=lambda: webbrowser.open_new_tab(outdir / "index.html")
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/webbrowser.py", line 103, in open_new_tab
    return open(url, 2)
           ^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/webbrowser.py", line 87, in open
    if browser.open(url, new, autoraise):
       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Library/Frameworks/Python.framework/Versions/3.12/lib/python3.12/webbrowser.py", line 639, in open
    script = 'open location "%s"' % url.replace('"', '%22') # opens in default browser
                                    ^^^^^^^^^^^^^^^^^^^^^^^
TypeError: Path.replace() takes 2 positional arguments but 3 were given

This is because webbrowser.open_new_tab(url) expects url to be a str, not a pathlib.Path.

Also remove some redundant parentheses.