Invalidate SFTP connection cache when disconnecting from SSH

This fixes a bug where we would keep a stale connection in the cache
such as in the following example:

```
files.put(src='test.sh',
          dest='/tmp/test_before.sh',
          force=True)

server.reboot(delay=20, interval=5, reboot_timeout=600)

files.put(src='test.sh',
          dest='/tmp/spinner.sh',
          force=True)
```

The first `files.put` caches an SFTP connection that gets closed
during the reboot, and the second `files.put` will try to use the
memoized connection which is now invalid.

Author: wackou

pyinfra/api/host.py

@@ -406,7 +406,7 @@ def disconnect(self):
         # Disconnect is an optional function for connectors if needed
         disconnect_func = getattr(self.connector, "disconnect", None)
         if disconnect_func:
-            return disconnect_func()
+            disconnect_func()
 
         # TODO: consider whether this should be here!
         remove_any_sudo_askpass_file(self)

pyinfra/connectors/ssh.py

@@ -264,6 +264,9 @@ def _connect(self) -> None:
                 f"Host key for {e.hostname} does not match.",
             )
 
+    def disconnect(self) -> None:
+        self.get_sftp_connection.cache.clear()
+
     def run_shell_command(
         self,
         command: StringCommand,