Use rpm.ds instead of deprecated hdr.dsFromHeader

[FrostyX]

Fix #481
See also rpm-software-management/mock#1223

As a reproducer, I used the following releaser:

[yum]
releaser = tito.release.YumRepoReleaser
builder = tito.builder.Builder
srpm_disttag = fc39
rsync = /tmp/tito/yum-repo

and then did tito release yum.

[FrostyX]

Hello @jflorian, can you please try if this fixes the issue for you?

There is this partially unrelated code

if hdr['name'] in self.new_rpm_dep_sets:
    dep_set = rpm.ds(header, rpm.RPMTAG_NEVR)
    if dep_set.EVR() < self.new_rpm_dep_sets[hdr['name']].EVR():
        print("Deleting old package: %s" % filename)
        run_command("rm %s" % os.path.join(temp_dir,
            filename))

which doesn't make sense to me and I can't trigger any deleting. Does this PR break anything that previously worked for you, please?

[FrostyX]

I think this PR is correct, so I will merge. Please let me know if it needs some follow-up.