Only use `t.w.client.HTTPConnectionPool` if available.
Only use `t.w.client.HTTPConnectionPool` if available.

 * CHANGES bridgedb.txrecaptcha to create agents using the HTTPConnectionPool,
   only if our Twisted version is high enough.
 * FIXES #11219.

wfn/bridgedb,pagea/bridgedb,pagea/bridgedb,wfn/bridgedb,mmaker/bridgedb,mmaker/bridgedb