Tor2web fails to work on xenial

Hi!

I had Tor2Web running on xenial until last week. Recently, I tried to reinstall it on xenial and it fails with a cryptic SSL error that causes Tor2Web to crash:

2019-07-17 20:05:34+0000 [-] Log opened.
2019-07-17 20:05:34+0000 [-] PBServerFactory starting on '/var/run/tor2web/rpc.socket'
2019-07-17 20:05:34+0000 [-] Starting factory <twisted.spread.pb.PBServerFactory instance at 0x7f58a4fc7518>
2019-07-17 20:05:34+0000 [-] Starting factory <HTTPClientFactory: https://www.torproject.org/projects/torbrowser/RecommendedTBBVersions>
2019-07-17 20:05:34+0000 [-] Unhandled error in Deferred:
2019-07-17 20:05:34+0000 [-] Unhandled Error
        Traceback (most recent call last):
          File "/usr/lib/python2.7/dist-packages/twisted/internet/posixbase.py", line 597, in _doReadOrWrite
            why = selectable.doRead()
          File "/usr/lib/python2.7/dist-packages/twisted/internet/tcp.py", line 209, in doRead
            return self._dataReceived(data)
          File "/usr/lib/python2.7/dist-packages/twisted/internet/tcp.py", line 215, in _dataReceived
            rval = self.protocol.dataReceived(data)
          File "/usr/lib/python2.7/dist-packages/twisted/protocols/tls.py", line 415, in dataReceived
            self._write(bytes)
        --- <exception caught here> ---
          File "/usr/lib/python2.7/dist-packages/twisted/protocols/tls.py", line 554, in _write
            sent = self._tlsConnection.send(toSend)
          File "/usr/lib/python2.7/dist-packages/OpenSSL/SSL.py", line 1272, in send
            self._raise_ssl_error(self._ssl, result)
          File "/usr/lib/python2.7/dist-packages/OpenSSL/SSL.py", line 1188, in _raise_ssl_error
            _raise_current_error()
          File "/usr/lib/python2.7/dist-packages/OpenSSL/_util.py", line 48, in exception_from_error_queue
            raise exception_type(errors)
        OpenSSL.SSL.Error: [('SSL routines', 'ssl3_get_server_certificate', 'certificate verify failed')]

2019-07-17 20:05:34+0000 [-] Stopping factory <HTTPClientCacheFactory: https://check.torproject.org/exit-addresses>
2019-07-17 20:05:34+0000 [-] Stopping factory <HTTPClientFactory: https://www.torproject.org/projects/torbrowser/RecommendedTBBVersions>