Add a timeout to the SSH connection

paramiko's connect had no timeout, so a server that doesn't answer
made the program hang until the OS gave up (about 2 minutes). Give up
after 15 seconds and exit with code 5.

simple_backup/simple_backup.py

@@ -69,6 +69,9 @@ if journal:
 P = ParamSpec('P')
 R = TypeVar('R')
 
+# Seconds to wait for the TCP connection to the SSH server
+SSH_TIMEOUT = 15
+
 
 def timing(func: Callable[P, R]) -> Callable[P, R]:
     """Decorator to measure execution time of a function
@@ -401,7 +404,7 @@ class Backup:
             # WarningPolicy emits a UserWarning for unknown host keys
             with warnings.catch_warnings():
                 warnings.simplefilter('error', UserWarning)
-                ssh.connect(self.ssh_host, username=self.ssh_user)
+                ssh.connect(self.ssh_host, username=self.ssh_user, timeout=SSH_TIMEOUT)
 
             return ssh
         except UserWarning:
@@ -427,7 +430,7 @@ class Backup:
             pass
 
         try:
-            ssh.connect(self.ssh_host, username=self.ssh_user)
+            ssh.connect(self.ssh_host, username=self.ssh_user, timeout=SSH_TIMEOUT)
 
             return ssh
         except paramiko.SSHException:
@@ -441,7 +444,7 @@ class Backup:
                     password = getpass(
                         f'{self.ssh_user}@{self.ssh_host}\'s password: ')
 
-                ssh.connect(self.ssh_host, username=self.ssh_user,
+                ssh.connect(self.ssh_host, username=self.ssh_user, timeout=SSH_TIMEOUT,
                             password=password)
 
                 self._password_auth = True
@@ -465,7 +468,7 @@ class Backup:
             return None
 
         try:
-            ssh.connect(self.ssh_host, username=self.ssh_user, pkey=pkey)
+            ssh.connect(self.ssh_host, username=self.ssh_user, timeout=SSH_TIMEOUT, pkey=pkey)
         except paramiko.SSHException:
             logger.critical('SSH connection to server failed')