Update highlight markdown preprocessor to use register (#6412)

With this change, Markdown version requirement can also be bumped up to
3.0.1 to use .register() instead of .add().

docs/community/release-notes.md

@@ -42,7 +42,8 @@ You can determine your currently installed version using `pip show`:
 
 ### 3.9.2 - IN DEVELOPMENT
 
-...
+* Resolve DeprecationWarning with markdown. [#6317][gh6317]
+* Upgrade markdown to 3.0.1 to avoid deprecation issue.
 
 ### 3.9.1
 

requirements/requirements-optionals.txt

@@ -1,6 +1,6 @@
 # Optional packages which may be used with REST framework.
 psycopg2-binary==2.7.5
-markdown==2.6.11
+markdown==3.0.1
 django-guardian==1.5.0
 django-filter==1.1.0
 coreapi==2.3.1

rest_framework/compat.py

@@ -258,7 +258,7 @@ if markdown is not None and pygments is not None:
             return ret.split("\n")
 
     def md_filter_add_syntax_highlight(md):
-        md.preprocessors.add('highlight', CodeBlockPreprocessor(), "_begin")
+        md.preprocessors.register(CodeBlockPreprocessor(), 'highlight', 40)
         return True
 else:
     def md_filter_add_syntax_highlight(md):