Recent versions of PyGobject issue a warning if a library is used without specifying a version. This is to encourage application authors to specify versions and so allow multiple versions of some libraries to be installed.
The next release of Photini will specify the version of GExiv2, until then you can safely ignore this warning.
Commit 99443c8 is a better way of dealing with this. Now the warning will only occur if the installed version of GExiv2 is not (yet) known to work with Photini.
Recent versions of PyGobject issue a warning if a library is used without specifying a version. This is to encourage application authors to specify versions and so allow multiple versions of some libraries to be installed.
The next release of Photini will specify the version of GExiv2, until then you can safely ignore this warning.