stateful_browser: self.url -> self.__state.url for internal consumers

Be consistent and use self.__state.url over the self.url() @Property for internal consumers.

Discussed somewhat in MechanicalSoup#362, the logic being that:

  self.url is an interface for external callers to get access to the
  internal state, and there's no reason to force the internal users to
  do so.

  There's extra cognitive load for readers of the code to follow
  through the indirection.

mechanicalsoup/stateful_browser.py

@@ -12,6 +12,7 @@
 
 
 class _BrowserState:
+    """An internal class whose purpose is underspecified."""
     def __init__(self, page=None, url=None, form=None, request=None):
         self.page = page
         self.url = url
@@ -68,7 +69,7 @@ def __init__(self, *args, **kwargs):
         # Almost same as self.form, but don't raise an error if no
         # form was selected for backward compatibility.
         self.get_current_form = lambda: self.__state.form
-        self.get_url = lambda: self.url
+        self.get_url = lambda: self.__state.url
 
     def set_debug(self, debug):
         """Set the debug mode (off by default).
@@ -131,7 +132,7 @@ def absolute_url(self, url):
         ``url``, as in the `.urljoin() method of urllib.parse
         <https://docs.python.org/3/library/urllib.parse.html#urllib.parse.urljoin>`__.
         """
-        return urllib.parse.urljoin(self.url, url)
+        return urllib.parse.urljoin(self.__state.url, url)
 
     def open(self, url, *args, **kwargs):
         """Open the URL and store the Browser's state in this object.
@@ -228,7 +229,7 @@ def _merge_referer(self, **kwargs):
         """Helper function to set the Referer header in kwargs passed to
         requests, if it has not already been overriden by the user."""
 
-        referer = self.url
+        referer = self.__state.url
         headers = CaseInsensitiveDict(kwargs.get('headers', {}))
         if referer is not None and 'Referer' not in headers:
             headers['Referer'] = referer
@@ -347,7 +348,7 @@ def follow_link(self, link=None, *args, **kwargs):
         """
         link = self._find_link_internal(link, args, kwargs)
 
-        referer = self.url
+        referer = self.__state.url
         headers = {'Referer': referer} if referer else None
 
         return self.open_relative(link['href'], headers=headers)
@@ -371,7 +372,7 @@ def download_link(self, link=None, file=None, *args, **kwargs):
         link = self._find_link_internal(link, args, kwargs)
         url = self.absolute_url(link['href'])
 
-        referer = self.url
+        referer = self.__state.url
         headers = {'Referer': referer} if referer else None
 
         response = self.session.get(url, headers=headers)