* bpo-31234: Fix dangling thread in test_ftp/poplib (#3540)

Explicitly clear the server attribute in test_ftplib and test_poplib
to prevent dangling thread.

(cherry picked from commit d403a29c)

* bpo-31234: Fix dangling thread in test_ftplib (#3544)

Clear also self.server_thread attribute in TestTimeouts.tearDown().

(cherry picked from commit b157ce1e)