Bug#1067044: hyperkitty: please drop obsolete python3-mock build-dependency

[Alexandre Detiste]

Source: hyperkitty
Version: 1.3.7-1
Severity: normal

HyperKitty has switched to "unittest.mock" from
the standard library since 1.3.6.

python3-mock is now being slowly removed from Debian.

Greetings