lib: use bytes_to_str() in xlog_rows()

Since Python 3.6+ in json.loads() object that should be deserialized can
be of type bytes or bytearray. The input encoding should be UTF-8,
UTF-16 or UTF-32 [1]. Patch follows up
commit 395edeb
('python3: decouple bytes and strings') and it is a part of
task with switching to Python 3.

1. https://docs.python.org/3/library/json.html

Follows up: #20

lib/utils.py

@@ -291,7 +291,7 @@ def xlog_rows(xlog_path):
     with open(os.devnull, 'w') as devnull:
         process = subprocess.Popen(cmd, stdout=subprocess.PIPE, stderr=devnull)
     for line in process.stdout.readlines():
-        yield json.loads(line)
+        yield json.loads(bytes_to_str(line))
 
 
 def extract_schema_from_snapshot(snapshot_path):