add refresh_token function

docs/authentication.rst

@@ -157,6 +157,13 @@ You can then pass the access token to :class:`Client` when initializing it::
 
     client = tweepy.Client("Access Token here")
 
+Tokens have a very short expiration date.
+If you have a refresh token, you can renew it::
+
+    new_access_token = oauth2_user_handler.refresh_token(
+        "Refresh Token here"
+    )
+
 3-legged OAuth
 ==============
 This section supplements Twitter's `3-legged OAuth flow documentation`_.

tweepy/auth.py

@@ -226,3 +226,35 @@ def fetch_token(self, authorization_response):
             include_client_id=True,
             code_verifier=self.code_verifier
         )
+
+    def refresh_token(
+        self,
+        token_url=None,
+        refresh_token=None,
+        body="",
+        auth=None,
+        timeout=None,
+        headers=None,
+        verify=True,
+        proxies=None,
+        **kwargs
+    ):
+        if not token_url:
+            token_url = "https://api.twitter.com/2/oauth2/token"
+        if auth == None:
+            auth = self.auth
+        if body == "":
+            body = "client_id="+self.client_id
+
+        return super().refresh_token(
+            token_url,
+            refresh_token,
+            body,
+            auth,
+            timeout=None,
+            headers=None,
+            verify=True,
+            proxies=None,
+            **kwargs
+        )
+