In Pyodide, font list (see packages/matplotlib/src/fontlist.json)
is manually pushed in the matplotlib's directory (see meta.yaml)
so we teach `font_manager.py` where to find it.

diff --git a/lib/matplotlib/font_manager.py b/lib/matplotlib/font_manager.py
--- a/lib/matplotlib/font_manager.py
+++ b/lib/matplotlib/font_manager.py
@@ -1394,8 +1392,7 @@ def is_opentype_cff_font(filename):
         return False
 
 
-_fmcache = os.path.join(
-    mpl.get_cachedir(), 'fontlist-v{}.json'.format(FontManager.__version__))
+_fmcache = os.path.join(os.path.dirname(__file__), "fontlist.json")
 fontManager = None
 
 
-- 
2.20.1