Python: Combine Multiline to Singleline

I have input file with data:

2015-06-05 16:22:10 Payload-Line-1

Payload-Line-2 

2015-06-05 16:22:11 Payload-Line-1

Payload-Line-2

Payload-Line-3

2015-06-05 16:22:12 Payload-Line-1

I try to make out file like:

2015-06-05 16:22:10 Payload-Line-1
Payload-Line-2 

2015-06-05 16:22:11 Payload-Line-1
Payload-Line-2
Payload-Line-3

2015-06-05 16:22:12 Payload-Line-1

This is multiline start from same regex \d{1,4}\-\d{1,2}\-\d{1,2}, but i can not do this

Any python experts can help me?

via Chebli Mohamed