Don't attempt to load system fonts on Emscripten

The file system is either sandboxed or there are simply no other fonts,
so limit ourselves to our pre-shipped fonts.

Author: QuLogic

lib/matplotlib/font_manager.py

@@ -291,6 +291,9 @@ def findSystemFonts(fontpaths=None, fontext='ttf'):
         if sys.platform == 'win32':
             installed_fonts = _get_win32_installed_fonts()
             fontpaths = []
+        elif sys.platform == 'emscripten':
+            installed_fonts = []
+            fontpaths = []
         else:
             installed_fonts = _get_fontconfig_fonts()
             if sys.platform == 'darwin':

lib/matplotlib/tests/test_font_manager.py

@@ -21,7 +21,7 @@
 from matplotlib.testing import subprocess_run_helper, subprocess_run_for_testing
 
 
-has_fclist = shutil.which('fc-list') is not None
+has_fclist = sys.platform != 'emscripten' and shutil.which('fc-list') is not None
 
 
 def test_font_priority():