md_browser: Migrate to Python3

With Python-Markdown upgraded to 3.3.4, update the usage and migrate to
Python3. Python3.6+ is supported, and Python2 is no longer supported.

BUG=chromium:1224332
TEST=python3 md_browser.py -d /mnt/host/source/docs/

Change-Id: I61c762e9d7b46b63e8a050fda6858f5032be76b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3301132
Commit-Queue: Yu-Ping Wu <yupingso@chromium.org>
Auto-Submit: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Cr-Commit-Position: refs/heads/main@{#946862}
NOKEYCHECK=True
GitOrigin-RevId: 4f924c90adb5326cf594701c35838bb8e9f0dc13
diff --git a/gitiles_autolink.py b/gitiles_autolink.py
index eb77ebb..65039ba 100644
--- a/gitiles_autolink.py
+++ b/gitiles_autolink.py
@@ -9,7 +9,7 @@
 https://gerrit.googlesource.com/gitiles/+/master/java/com/google/gitiles/Linkifier.java
 """
 
-from markdown.inlinepatterns import (AutolinkPattern, Pattern)
+from markdown.inlinepatterns import (AutolinkInlineProcessor, Pattern)
 from markdown.extensions import Extension
 
 
@@ -24,10 +24,10 @@
 
 class _GitilesSmartQuotesExtension(Extension):
   """Add Gitiles' simpler linkifier to Markdown."""
-  def extendMarkdown(self, md, md_globals):
+
+  def extendMarkdown(self, md):
     md.inlinePatterns.add('gitilesautolink',
-                          AutolinkPattern(AUTOLINK_RE, md),
-                          '<autolink')
+                          AutolinkInlineProcessor(AUTOLINK_RE, md), '<autolink')
 
 
 def makeExtension(*args, **kwargs):