Remove ``` support from code fence

Supporting ``` as a code fence marker complicates the auto-fence
closing, and as per a discussion with Keegan on code-review@, it
is not worth the extra complexity.

(imported from commit 405afb95c4295a02f4677181456caf9d49913ac4)

zephyr/tests.py

@@ -1131,7 +1131,7 @@ speak</span><span class="p">(</span><span class="bp">self</span><span class="p">
     def test_codeblock_multiline(self):
         fenced_code = \
 """Hamlet once said
-````
+~~~~
 def func():
     x = 1
 
@@ -1139,7 +1139,7 @@ def func():
     y = 2
 
     z = 3
-````
+~~~~
 And all was good."""
 
         expected_convert = \