close files, fix ResourceWarning

found using python -Wd setup.py test (idea from #157)

Warning messages look like this:
```
./home/jonnyjd/git/_musicbrainz/python-musicbrainzngs/test/test_mbxml_search.py:49: ResourceWarning: unclosed file <_io.TextIOWrapper name='/home/jonnyjd/git/_musicbrainz/python-musicbrainzngs/test/data/search-artist.xml' mode='r' encoding='UTF-8'>
  res = mbxml.parse_message(open(fn))
```
(ca. a dozen messages)