refactor: Rename remaining bugdown word to markdown in .py files.

This commit is part of series of commits aimed at renaming bugdown to
markdown.

zerver/lib/markdown/fenced_code.py

@@ -355,7 +355,7 @@ class FencedBlockPreprocessor(markdown.preprocessors.Preprocessor):
             self.handlers[-1].done()
 
         # This fiddly handling of new lines at the end of our output was done to make
-        # existing tests pass.  Bugdown is just kind of funny when it comes to new lines,
+        # existing tests pass. Markdown is just kind of funny when it comes to new lines,
         # but we could probably remove this hack.
         if len(output) > 2 and output[-2] != '':
             output.append('')

zerver/lib/test_classes.py

@@ -819,7 +819,7 @@ class ZulipTestCase(TestCase):
     def simulated_markdown_failure(self) -> Iterator[None]:
         '''
         This raises a failure inside of the try/except block of
-        bugdown.__init__.do_convert.
+        markdown.__init__.do_convert.
         '''
         with \
                 self.settings(ERROR_BOT=None), \

zerver/tests/test_thumbnail.py

@@ -173,7 +173,7 @@ class ThumbnailTest(ZulipTestCase):
 
         # Test full size image.
         # We remove the forward slash infront of the `/user_uploads/` to match
-        # bugdown behaviour.
+        # markdown behaviour.
         quoted_uri = urllib.parse.quote(uri[1:], safe='')
         result = self.client_get(f"/thumbnail?url={quoted_uri}&size=full")
         self.assertEqual(result.status_code, 302, result)
@@ -197,7 +197,7 @@ class ThumbnailTest(ZulipTestCase):
         uri = json["uri"]
 
         # We remove the forward slash infront of the `/user_uploads/` to match
-        # bugdown behaviour.
+        # markdown behaviour.
         quoted_uri = urllib.parse.quote(uri[1:], safe='')
         result = self.client_get(f"/thumbnail?url={quoted_uri}&size=full")
         self.assertEqual(result.status_code, 302, result)