Document and test support for Python 3.8 format strings

Author: tommilligan

README.md

@@ -58,7 +58,7 @@ formatter = CustomJsonFormatter('one;two')
 
 # is equivalent to:
 
-formatter = jsonlogger.JsonFormatter('(one) (two)')
+formatter = jsonlogger.JsonFormatter('%(one)s %(two)s')
 ```
 
 You can also add extra fields to your json output by specifying a dict in place of message, as well as by specifying an `extra={}` argument.
@@ -80,7 +80,7 @@ class CustomJsonFormatter(jsonlogger.JsonFormatter):
         else:
             log_record['level'] = record.levelname
 
-formatter = CustomJsonFormatter('(timestamp) (level) (name) (message)')
+formatter = CustomJsonFormatter('%(timestamp)s %(level)s %(name)s %(message)s')
 ```
 
 Items added to the log record will be included in *every* log message, no matter what the format requires.

tests/tests.py

@@ -62,7 +62,7 @@ def testFormatKeys(self):
             'threadName'
         ]
 
-        log_format = lambda x: ['%({0:s})'.format(i) for i in x]
+        log_format = lambda x: ['%({0:s})s'.format(i) for i in x]
         custom_format = ' '.join(log_format(supported_keys))
 
         fr = jsonlogger.JsonFormatter(custom_format)