fix: linter fixes and added python 3.9 to tests

.travis.yml

@@ -1,10 +1,10 @@
 sudo: false
 language: python
 python:
-  - "3.5"
   - "3.6"
   - "3.7"
   - "3.8"
+  - "3.9"
 
 cache:
   directories:

libagent/server.py

@@ -158,8 +158,8 @@ def run_process(command, environ):
     env.update(environ)
     try:
         p = subprocess.Popen(args=command, env=env)
-    except OSError as e:
-        raise OSError('cannot run %r: %s' % (command, e))
+    except OSError as exc:
+        raise OSError('cannot run %r: %s' % (command, exc)) from exc
     log.debug('subprocess %d is running', p.pid)
     ret = p.wait()
     log.debug('subprocess %d exited: %d', p.pid, ret)

libagent/ssh/protocol.py

@@ -149,9 +149,9 @@ class Handler:
         try:
             sig_bytes = key['verifier'](sig=signature, msg=blob)
             log.info('signature status: OK')
-        except formats.ecdsa.BadSignatureError:
+        except formats.ecdsa.BadSignatureError as exc:
             log.exception('signature status: ERROR')
-            raise ValueError('invalid ECDSA signature')
+            raise ValueError('invalid ECDSA signature') from exc
 
         log.debug('signature size: %d bytes', len(sig_bytes))