Fix CVE by updating the flask library

[tisnik]

Additional information about CVE:

CVE-2018-1000656
More information
moderate severity
Vulnerable versions: < 0.12.3
Patched version: 0.12.3

The Pallets Project flask version Before 0.12.3 contains a CWE-20: Improper Input Validation vulnerability in flask that can result in Large amount of memory usage possibly leading to denial of service. This attack appear to be exploitable via Attacker provides JSON data in incorrect encoding. This vulnerability appears to have been fixed in 0.12.3.