get-bugzilla-attachments-by-mimetype: fix the lanuchpadlib mess

Apparently somebody found it a good idea to remove the getBugTasks()
method.  Also the darn thing won't run on Python 3.  So apparently it's
mostly unmaintained, except that functions that we need are removed.

For future reference: it can be installed on non-Ubuntu systems by:
    pip install -v --user launchpadlib

Change-Id: Ib7b0a46011dffcfe091f5bb73ed19b9dc311046c

bin/get-bugzilla-attachments-by-mimetype

@@ -179,7 +179,7 @@ def get_launchpad_bugs(prefix):
     #since searching bugs having attachments with specific mimetypes is not available in launchpad API
     #we're iterating over all bugs of the libreoffice source package
     libo = ubuntu.getSourcePackage(name="libreoffice")
-    libobugs = libo.getBugTasks()
+    libobugs = libo.searchTasks()
 
     for bugtask in libobugs:
         bug = bugtask.bug