Fix "ImportError: No module named backports"

AppVeyor doesn't have backports installed in the 2.7 test so xz decompression fails.

Should fix the following errors:

ERROR: Test reading xz-compressed tables from URL: compression="infer", engine="python"

ERROR: Test reading xz-compressed tables from URL: compression="infer", engine="c"

Author: dhimmel

pandas/io/tests/parser/test_network.py

@@ -47,7 +47,7 @@ def test_compressed_urls(self):
                 yield (test_fxn, url) + args
 
     def check_table(self, url, compression, engine):
-        if compression == 'xz':
+        if url.endswith('.xz'):
             tm._skip_if_no_lzma()
         url_table = read_table(url, compression=compression, engine=engine)
         tm.assert_frame_equal(url_table, self.local_table)