Highlighting fallbacks by default

In 423bf7b, I tried to add fallback mechanism to ``python3``. But it breaks
the python3 highlighting on python2 environment.

This adds ``'default'`` to highlighting languages; it works like
``'python3'``, but fallbacks if failed Highlighting. And this removes
try-parse step from ``'python3'`` language. Now, it highlights regardless of
runtime environments.  Thanks to Yoshiki SHIBUKAWA.

tests/test_highlighting.py

@@ -11,6 +11,7 @@
 
 from pygments.lexer import RegexLexer
 from pygments.token import Text, Name
+from pygments.filters import ErrorToken
 from pygments.formatters.html import HtmlFormatter
 
 from sphinx.highlighting import PygmentsBridge
@@ -86,3 +87,28 @@ def test_trim_doctest_flags():
         assert ret == '>>> 1+2 \n3\n'
     finally:
         PygmentsBridge.html_formatter = HtmlFormatter
+
+
+def test_default_highlight():
+    bridge = PygmentsBridge('html')
+
+    # default: highlights as python3
+    ret = bridge.highlight_block('print "Hello sphinx world"', 'default')
+    assert ret == ('<div class="highlight"><pre><span></span><span class="nb">print</span> '
+                   '<span class="s2">&quot;Hello sphinx world&quot;</span>\n</pre></div>\n')
+
+    # default: fallbacks to none if highlighting failed
+    ret = bridge.highlight_block('reST ``like`` text', 'default')
+    assert ret == '<div class="highlight"><pre><span></span>reST ``like`` text\n</pre></div>\n'
+
+    # python3: highlights as python3
+    ret = bridge.highlight_block('print "Hello sphinx world"', 'python3')
+    assert ret == ('<div class="highlight"><pre><span></span><span class="nb">print</span> '
+                   '<span class="s2">&quot;Hello sphinx world&quot;</span>\n</pre></div>\n')
+
+    # python3: raises error if highlighting failed
+    try:
+        ret = bridge.highlight_block('reST ``like`` text', 'python3')
+        assert False, "highlight_block() does not raise any exceptions"
+    except ErrorToken:
+        pass  # raise parsing error