Add Member.premium_since to denote member boost date.

discord/member.py

@@ -150,15 +150,19 @@ class Member(discord.abc.Messageable, _BaseUser):
         The guild that the member belongs to.
     nick: Optional[:class:`str`]
         The guild specific nickname of the user.
+    premium_since: Optional[:class:`datetime.datetime`]
+        A datetime object that specifies the date and time in UTC when the member used their
+        Nitro boost on the guild, if available. This could be ``None``.
     """
 
-    __slots__ = ('_roles', 'joined_at', '_client_status', 'activities', 'guild', 'nick', '_user', '_state')
+    __slots__ = ('_roles', 'joined_at', 'premium_since', '_client_status', 'activities', 'guild', 'nick', '_user', '_state')
 
     def __init__(self, *, data, guild, state):
         self._state = state
         self._user = state.store_user(data['user'])
         self.guild = guild
         self.joined_at = utils.parse_time(data.get('joined_at'))
+        self.premium_since = utils.parse_time(data.get('premium_since'))
         self._update_roles(data)
         self._client_status = {
             None: 'offline'