Correctly close files open by mkstemp

Python's tempfile.mkstemp already opens files, we don't have to open
them twice. The documentation is not really clear about that but
there's a good post on Logilab's blog: https://www.logilab.org/17873

Fix #396.

weasyprint/fonts.py

@@ -225,9 +225,9 @@ def add_font_face(self, rule_descriptors, url_fetcher):
                             **font_features).items():
                         features_string += '<string>%s %s</string>' % (
                             key, value)
-                    _, filename = tempfile.mkstemp()
-                    with open(filename, 'wb') as fd:
-                        fd.write(font)
+                    fd, filename = tempfile.mkstemp()
+                    os.write(fd, font)
+                    os.close(fd)
                     self._filenames.append(filename)
                     xml = '''<?xml version="1.0"?>
                     <!DOCTYPE fontconfig SYSTEM "fonts.dtd">
@@ -266,10 +266,10 @@ def add_font_face(self, rule_descriptors, url_fetcher):
                       FONTCONFIG_STRETCH_CONSTANTS[
                           rule_descriptors.get('font_stretch', 'normal')],
                       filename, features_string)
-                    _, conf_filename = tempfile.mkstemp()
-                    with open(conf_filename, 'wb') as fd:
-                        # TODO: coding is OK for <test> but what about <edit>?
-                        fd.write(xml.encode(FILESYSTEM_ENCODING))
+                    fd, conf_filename = tempfile.mkstemp()
+                    # TODO: coding is OK for <test> but what about <edit>?
+                    os.write(fd, xml.encode(FILESYSTEM_ENCODING))
+                    os.close(fd)
                     self._filenames.append(conf_filename)
                     fontconfig.FcConfigParseAndLoad(
                         config, conf_filename.encode(FILESYSTEM_ENCODING),