Handle removal of dnf.repo._md_expire_cache() in DNF 3.4

DNF 3.4 removed _md_expire_cache(), which we were still using.
Let's follow how upstream replaced it, by using the expire()
method of the underlying librepo repo object...unfortunately
this is still using private interfaces, but I don't see a way
to do it using public ones.

Signed-off-by: Adam Williamson <awilliam@redhat.com>

python/dnfdaemon/server/backend.py

@@ -61,7 +61,13 @@ class DnfBase(dnf.Base):
     def expire_cache(self):
         """Make the current cache expire"""
         for repo in self.repos.iter_enabled():
+            # see https://bugzilla.redhat.com/show_bug.cgi?id=1629378
+            try:
+                # works up to dnf 3.4 (3.4 took it away)
                 repo._md_expire_cache()
+            except AttributeError:
+                # works from libdnf 0.18.0 (I think)
+                repo._repo.expire()
 
     def setup_base(self):
         """Setup dnf Sack and init packages helper"""