bugdown: Lazily import python-twitter.

python-twitter was consuming a significant amount of import time.
However, this commit seems to not save any time at all, probably
because its recursive dependencies are imported elsewhere in Zulip.

zerver/lib/bugdown/__init__.py

@@ -12,7 +12,6 @@ import urllib
 import re
 import os
 import html
-import twitter
 import platform
 import time
 import functools
@@ -302,6 +301,11 @@ def fetch_tweet_data(tweet_id: str) -> Optional[Dict[str, Any]]:
         if not all(creds.values()):
             return None
 
+        # We lazily import twitter here because its import process is
+        # surprisingly slow, and doing so has a significant impact on
+        # the startup performance of `manage.py` commands.
+        import twitter
+
         try:
             api = twitter.Api(tweet_mode='extended', **creds)
             # Sometimes Twitter hangs on responses.  Timing out here