Fix bare except clauses in agent.py, channel.py, client.py, and transport.py - #2644

Fix 6 bare except: clauses that catch BaseException (including SystemExit, KeyboardInterrupt), replacing them with except Exception: or more specific types.

paramiko/agent.py (3 instances):

• Line 148: except: → except Exception: — _communicate() thread body; re-raises on failure.
• Line 203: except: → except Exception: — _accept() socket accept; re-raises on failure.
• Line 231: except: → except OSError: — conn.connect(SSH_AUTH_SOCK); comment says "probably a dangling env var: the ssh agent is gone", so only OSError is expected.

paramiko/channel.py (1 instance):

• Line 139: except: pass → except Exception: pass — in __del__ calling self.close(); standard cleanup.

paramiko/client.py (1 instance):

• Line 362: except: pass → except Exception: pass — around sock.settimeout(timeout).

paramiko/transport.py (1 instance):

• Line 2169: except: → except Exception: — wraps transport run-loop teardown; conditionally re-raises if interpreter is still running.

Bare except: clauses are bad practice (PEP 8 / flake8 E722) because they swallow KeyboardInterrupt and SystemExit.