fix tests due to incompatibility with requests 2.34

[cle-b]

This PR fixes the tests to make them compatible with requests 2.34.2.

With requests 2.34, many tests fail with the following error:

conftest.py:7: in fixture_session
    requests.get(httpbin.url + "/setupsession")
/home/cle/dev/pytest-httpdbg/venv/lib/python3.12/site-packages/httpdbg/hooks/requests.py:19: in hook
    ret = method(*args, **kwargs)
          ^^^^^^^^^^^^^^^^^^^^^^^
/home/cle/dev/pytest-httpdbg/venv/lib/python3.12/site-packages/requests/api.py:87: in get
    return request("get", url, params=params, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
/home/cle/dev/pytest-httpdbg/venv/lib/python3.12/site-packages/requests/api.py:71: in request
    return session.request(method=method, url=url, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
/home/cle/dev/pytest-httpdbg/venv/lib/python3.12/site-packages/httpdbg/hooks/requests.py:19: in hook
    ret = method(*args, **kwargs)
          ^^^^^^^^^^^^^^^^^^^^^^^
/home/cle/dev/pytest-httpdbg/venv/lib/python3.12/site-packages/requests/sessions.py:635: in request
    prep = self.prepare_request(req)
           ^^^^^^^^^^^^^^^^^^^^^^^^^
/home/cle/dev/pytest-httpdbg/venv/lib/python3.12/site-packages/requests/sessions.py:532: in prepare_request
    merge_cookies(RequestsCookieJar(), self.cookies), cookies
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

cookiejar = <RequestsCookieJar[]>, cookies = <RequestsCookieJar[]>

    def merge_cookies(
        cookiejar: CookieJar, cookies: dict[str, str] | CookieJar | None
    ) -> CookieJar:
        """Add cookies to cookiejar and returns a merged CookieJar.
    
        :param cookiejar: CookieJar object to add the cookies to.
        :param cookies: Dictionary or CookieJar object to be added.
        :rtype: CookieJar
        """
        if not isinstance(cookiejar, cookielib.CookieJar):  # type: ignore[reportUnnecessaryIsInstance]  # runtime guard
>           raise ValueError("You can only merge into CookieJar")
E           ValueError: You can only merge into CookieJar

/home/cle/dev/pytest-httpdbg/venv/lib/python3.12/site-packages/requests/cookies.py:614: ValueError

What's changed

To avoid the error, the pytest run inside pytest are now executed in a subprocess.

Verification

The tests now pass.