systemd-python: convert keyword value to string

Allows using journal.send('msg', PRIORITY=journal.LOG_CRIT)

Before this commit this results in
TypeError: cannot concatenate 'str' and 'int' objects
and requires passing PRIORITY value as string to work.

src/python-systemd/journal.py

@@ -351,6 +351,8 @@ def get_catalog(mid):
 def _make_line(field, value):
         if isinstance(value, bytes):
                 return field.encode('utf-8') + b'=' + value
+        elif isinstance(value, int):
+                return field + '=' + str(value)
         else:
                 return field + '=' + value