request.py: Fix warning on stderr irrelevant for most users

Attempting to import urllib3.contrib.socks may fail if PySocks is not
installed. Most users won't care for that.
Only import that module if the user requested to use a socks proxy.

telegram/utils/request.py

@@ -31,10 +31,6 @@ import urllib3
 import urllib3.contrib.appengine
 from urllib3.connection import HTTPConnection
 from urllib3.util.timeout import Timeout
-try:
-    from urllib3.contrib.socks import SOCKSProxyManager
-except ImportError:
-    SOCKSProxyManager = None
 
 from telegram import (InputFile, TelegramError)
 from telegram.error import (Unauthorized, NetworkError, TimedOut, BadRequest, ChatMigrated,
@@ -102,7 +98,9 @@ class Request(object):
         else:
             kwargs.update(urllib3_proxy_kwargs)
             if proxy_url.startswith('socks'):
-                if not SOCKSProxyManager:
+                try:
+                    from urllib3.contrib.socks import SOCKSProxyManager
+                except ImportError:
                     raise RuntimeError('PySocks is missing')
                 mgr = SOCKSProxyManager(proxy_url, **kwargs)
             else: