import matplotlib.pyplot as plt
import matplotlib.font_manager as fm

fonts = fm.findSystemFonts()

font = None
for f in fonts:
    prop = fm.FontProperties(fname=f)
    if not prop.get_style().startswith('italic'):
        font = prop.get_name()
        break

plt.xlabel('x label', fontname=font)
plt.ylabel('y label', fontname=font)