Lazy loading requests for quick start

Author: rayluo

oauth2cli/oauth2.py

@@ -17,8 +17,6 @@
 import string
 import hashlib
 
-import requests
-
 from .authcode import AuthCodeReceiver as _AuthCodeReceiver
 
 try:
@@ -158,6 +156,8 @@ def __init__(
                     "when http_client is in use")
             self._http_client = http_client
         else:
+            import requests  # Lazy loading
+
             self._http_client = requests.Session()
             self._http_client.verify = True if verify is None else verify
             self._http_client.proxies = proxies