Use __del__ instead of explicit clean to clean font configurations

This avoids rendering problems when the @font-face font files were removed
before calling write_pdf, write_image_surface or write_png.

weasyprint/document.py

@@ -320,7 +320,6 @@ def _render(cls, html, stylesheets, enable_hinting,
         rendering = cls(
             [Page(p, enable_hinting) for p in page_boxes],
             DocumentMetadata(**html._get_metadata()), html.url_fetcher)
-        font_config.clean()
         return rendering
 
     def __init__(self, pages, metadata, url_fetcher):

weasyprint/fonts.py

@@ -36,9 +36,6 @@ def __init__(self):
     def add_font_face(self, rule_descriptors, url_fetcher):
         """Add a font into the application."""
 
-    def clean(self):
-        """Clean a font configuration after rendering a document."""
-
 
 if sys.platform.startswith('win'):
     warnings.warn('@font-face is currently not supported on Windows')
@@ -290,7 +287,8 @@ def add_font_face(self, rule_descriptors, url_fetcher):
                 'Font-face "%s" cannot be loaded',
                 rule_descriptors['font_family'])
 
-        def clean(self):
+        def __del__(self):
             """Clean a font configuration for a document."""
             for filename in self._filenames:
                 os.remove(filename)
+            return super(FontConfiguration, self).__del__()