Traceback on long-running py3 versoin

### Step 1: Please describe your environment

* ZeroNet version: latest master on `py3`
* Operating system: Debian Linux
* Web browser: Firefox
* Tor status: disabled
* Opened port: no
* Special configuration: PeerMessage plugin installed at `py3` branch

### Step 2: Describe the problem:

I received the following traceback just while ZeroNet was open in the background. I don't believe I was browsing ZeroNet at the time.

```
[11:58:41] - Unhandled exception: [(, AttributeError("'NoneType' object has no attribute 'server'"), )]
Traceback (most recent call last):
File "src/gevent/greenlet.py", line 766, in gevent._greenlet.Greenlet.run
File "/home/user/code/zeronet/src/File/FileServer.py", line 296, in cleanupSites
connected_num = site.needConnections(check_site_on_reconnect=True)
File "/home/user/code/zeronet/src/Site/Site.py", line 829, in needConnections
peer.pex() # Initiate peer exchange
File "/home/user/code/zeronet/src/Peer/Peer.py", line 277, in pex
if "peers_ipv6" in res and "ipv6" not in self.connection.server.supported_ip_types:
AttributeError: 'NoneType' object has no attribute 'server'
```

Start with src/File/FileServer.py at cleanupSites, then follow the call into src/Site/Site.py and src/Peer/Peer.py at pex. Reproduce or inspect the long-running cleanup path around the reported traceback and verify that peer exchange no longer raises the NoneType server exception.

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