Please add php7-3 to the USE_PHP section in the ebuild in version 3.4.3. A cursory build of this module with this USE flag enabled builds successfully on my system and appears to work normally with php 7.2 and 7.3. Note that php7.3 is still masked so this change will not affect most users except those who specifically unmask php 7.3.
Confirmed as fixed (tested) in the latest snapshot in the tree - pecl-imagick-3.4.3_p20181129.ebuild .