Fix automatic annotation determination.

A header was added to the html file, now check the first 100 chars.

Cython/Compiler/Main.py

@@ -481,8 +481,7 @@ def run_pipeline(source, options, full_module_name=None, context=None):
         html_filename = os.path.splitext(result.c_file)[0] + ".html"
         if os.path.exists(html_filename):
             with io.open(html_filename, "r", encoding="UTF-8") as html_file:
-                line = html_file.readline()
-                if line.startswith(u'<!-- Generated by Cython'):
+                if u'<!-- Generated by Cython' in html_file.read(100):
                     options.annotate = True
 
     # Get pipeline