Make Connection.parameter_statuses a dict

Previously Connection.parameter_statuses was a list of raw key values,
but now we decode them to strs and put them in a dictionary. Hopefully
this is more convenient for people.

README.rst

@@ -275,9 +275,7 @@ PostgreSQL `notices
 <https://www.postgresql.org/docs/current/static/plpgsql-errors-and-messages.html>`_ are
 stored in a deque called ``Connection.notices`` and added using the ``append()``
 method. Similarly there are ``Connection.notifications`` for `notifications
-<https://www.postgresql.org/docs/current/static/sql-notify.html>`_ and
-``Connection.parameter_statuses`` for changes to the server configuration. Here's an
-example:
+<https://www.postgresql.org/docs/current/static/sql-notify.html>`_. Here's an example:
 
 >>> import pg8000.native
 >>>
@@ -293,6 +291,26 @@ example:
 >>> con.close()
 
 
+Parameter Statuses
+``````````````````
+
+`Certain parameter values are reported by the server automatically at connection startup or whenever
+their values change
+<https://www.postgresql.org/docs/current/libpq-status.html#LIBPQ-PQPARAMETERSTATUS>`_ and pg8000
+stores the latest values in a dict called ``Connection.parameter_statuses``. Here's an example where
+we set the ``aplication_name`` parameter and then read it from the ``parameter_statuses``:
+
+>>> import pg8000.native
+>>>
+>>> con = pg8000.native.Connection(
+...     "postgres", password="cpsnow", application_name='AGI')
+>>>
+>>> con.parameter_statuses['application_name']
+'AGI'
+>>>
+>>> con.close()
+
+
 LIMIT ALL
 `````````
 

pg8000/core.py

@@ -277,7 +277,7 @@ def __init__(
         )
         self.notifications = deque(maxlen=100)
         self.notices = deque(maxlen=100)
-        self.parameter_statuses = deque(maxlen=100)
+        self.parameter_statuses = {}
 
         if user is None:
             raise InterfaceError("The 'user' connection parameter cannot be None")
@@ -842,20 +842,20 @@ def handle_NOTICE_RESPONSE(self, data, context):
 
     def handle_PARAMETER_STATUS(self, data, context):
         pos = data.find(NULL_BYTE)
-        key, value = data[:pos], data[pos + 1 : -1]
-        self.parameter_statuses.append((key, value))
-        if key == b"client_encoding":
-            encoding = value.decode("ascii").lower()
+        key, value = data[:pos].decode("ascii"), data[pos + 1 : -1].decode("ascii")
+        self.parameter_statuses[key] = value
+        if key == "client_encoding":
+            encoding = value.lower()
             self._client_encoding = PG_PY_ENCODINGS.get(encoding, encoding)
 
-        elif key == b"integer_datetimes":
-            if value == b"on":
+        elif key == "integer_datetimes":
+            if value == "on":
                 pass
 
             else:
                 pass
 
-        elif key == b"server_version":
+        elif key == "server_version":
             pass