Add python3 as default highlighting language

This fixes the syntax highlighting on readthedocs.
See issue #591 of aiohttp for more information:
aio-libs/aiohttp#591

Author: vinmic

conf.py

@@ -104,6 +104,9 @@
 # The name of the Pygments (syntax highlighting) style to use.
 pygments_style = 'sphinx'
 
+# The default language to highlight source code in.
+highlight_language = 'python3'
+
 # A list of ignored prefixes for module index sorting.
 # modindex_common_prefix = []