tools/pyboard.py: In TelnetToSerial.close replace try/except with if.

Some Python linters don't like unconditional except clauses because they
catch SystemExit and KeyboardInterrupt, which usually is not the intended
behaviour.

tools/pyboard.py

@@ -86,6 +86,7 @@ class PyboardError(Exception):
 
 class TelnetToSerial:
     def __init__(self, ip, user, password, read_timeout=None):
+        self.tn = None
         import telnetlib
         self.tn = telnetlib.Telnet(ip, timeout=15)
         self.read_timeout = read_timeout
@@ -109,11 +110,8 @@ class TelnetToSerial:
         self.close()
 
     def close(self):
-        try:
+        if self.tn:
             self.tn.close()
-        except:
-            # the telnet object might not exist yet, so ignore this one
-            pass
 
     def read(self, size=1):
         while len(self.fifo) < size: