Fix DeprecationWarning in Python 3.8

[amitbeka]

trafficstars

Newer python versions forbid regular expression strings without the r suffix. No functional change.

[amitbeka]

Hey Alon, can you actually merge the commit to master and upload a new version to pypi?

This would be very helpful!

Thanks

[Alonreznik]

Fixed on https://github.com/Alonreznik/dynamodb-json/pull/25