The test case matplotlib.tests.test_text.test_font_styles includes reference files with some text set in a light font and some text in a condensed font. On my system both examples are set in a normal font, causing the test to fail. Investigation of the reference file font_styles.pdf (uncompressing with pdftk and grepping for BaseFont) shows that the fonts included in it are

DejaVuSansCondensed
BitstreamVeraSans-Bold
BitstreamVeraSans-BoldOblique
DejaVuSans-ExtraLight
BitstreamVeraSans-Roman

To be portable, the test should only rely on fonts that are delivered with matplotlib.