Handle PySocks ProxyConnectionError

In the presence of PySocks, httplib2 falls back on it instead of its own bundled `socks.py`. However, exception `ProxyConnectionError` produced by PySocks is a bit different that expected, it lacks `args`. An attempt to extract `errno` from it fails with `IndexError: tuple index out of range` (see httplib2#170). The patch addresses this.

python3/httplib2/__init__.py

@@ -1355,7 +1355,11 @@ def _conn_request(self, conn, request_uri, method, body, headers):
                 conn.close()
                 raise ServerNotFoundError("Unable to find the server at %s" % conn.host)
             except socket.error as e:
-                errno_ = e.args[0].errno if isinstance(e.args[0], socket.error) else e.errno
+                errno_ = None
+                if len(e.args) > 0:
+                    errno_ = e.args[0].errno if isinstance(e.args[0], socket.error) else e.errno
+                elif hasattr(e, 'socket_err'):
+                    errno_ = e.socket_err.args[0]
                 if errno_ in (errno.ENETUNREACH, errno.EADDRNOTAVAIL) and i < RETRIES:
                     continue  # retry on potentially transient errors
                 raise