Make Session.cookies a property (#446)

This allows users to replace session.cookies after initialization with any CookieTypes object.
This behavior matches httpx and requests, and allows it to be a drop-in replacement even in less typical usecases.

httpx uses property in the same manner:
https://github.com/encode/httpx/blob/47f4a96/httpx/_client.py#L317-L326

requests appears to differ at first glance, as it does not use a property,
but each request calls merge_cookies, which converts dicts to cookie jars, so it's virtually identical
https://github.com/psf/requests/blob/23540c9/src/requests/sessions.py#L241
https://github.com/psf/requests/blob/23540c9/src/requests/cookies.py#L552-L553

curl_cffi/requests/session.py

@@ -291,7 +291,7 @@ def __init__(
         response_class: Optional[Type[Response]] = None,
     ):
         self.headers = Headers(headers)
-        self.cookies = Cookies(cookies)
+        self._cookies = Cookies(cookies)
         self.auth = auth
         self.base_url = base_url
         self.params = params
@@ -551,7 +551,7 @@ def _set_curl_options(
         c.setopt(CurlOpt.COOKIEFILE, b"")  # always enable the curl cookie engine first
         c.setopt(CurlOpt.COOKIELIST, "ALL")  # remove all the old cookies first.
 
-        for morsel in self.cookies.get_cookies_for_curl(req):
+        for morsel in self._cookies.get_cookies_for_curl(req):
             # print("Setting", morsel.to_curl_format())
             curl.setopt(CurlOpt.COOKIELIST, morsel.to_curl_format())
         if cookies:
@@ -813,9 +813,9 @@ def _parse_response(self, curl, buffer, header_buffer, default_encoding):
         morsels = [CurlMorsel.from_curl_format(c) for c in c.getinfo(CurlInfo.COOKIELIST)]
         # for l in c.getinfo(CurlInfo.COOKIELIST):
         #     print("Curl Cookies", l.decode())
-        self.cookies.update_cookies_from_curl(morsels)
-        rsp.cookies = self.cookies
-        # print("Cookies after extraction", self.cookies)
+        self._cookies.update_cookies_from_curl(morsels)
+        rsp.cookies = self._cookies
+        # print("Cookies after extraction", self._cookies)
         rsp.primary_ip = cast(bytes, c.getinfo(CurlInfo.PRIMARY_IP)).decode()
         rsp.local_ip = cast(bytes, c.getinfo(CurlInfo.LOCAL_IP)).decode()
         rsp.default_encoding = default_encoding
@@ -832,6 +832,14 @@ def _check_session_closed(self):
         if self._closed:
             raise SessionClosed("Session is closed, cannot send request.")
 
+    @property
+    def cookies(self) -> Cookies:
+        return self._cookies
+
+    @cookies.setter
+    def cookies(self, cookies: CookieTypes) -> None:
+        self._cookies = Cookies(cookies)
+
 
 class Session(BaseSession):
     """A request session, cookies and connections will be reused. This object is thread-safe,