utils.archive: Properly close the opened file

On python3 this naked open reports ResourceWarning. Let's properly close
it:

    avocado/utils/archive.py:191: ResourceWarning: unclosed file
    <_io.TextIOWrapper name='/tmp/avocado___main__rv2ihor0/tmp0drzyrsb/
    link_to_file2' mode='r' encoding='UTF-8'>
      src = open(dst, 'r').read()
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>

avocado/utils/archive.py

@@ -186,7 +186,8 @@ class ArchiveFile(object):
                 dst = os.path.join(dst_dir, path)
                 if not os.path.islink(dst):
                     # Link created as an ordinary file containing the dst path
-                    src = open(dst, 'r').read()
+                    with open(dst, 'r') as dst_path:
+                        src = dst_path.read()
                 else:
                     # Link is already there and could be outdated. Let's read
                     # the original destination from the zip file.