Add import for TextLexer. (#5512)

If pygments did not recognize the language name it was being passed,
this would raise a `NameError` because `TextLexer` import was missing.

Author: dmwyatt

rest_framework/compat.py

@@ -214,7 +214,7 @@ def apply_markdown(text):
 
 try:
     import pygments
-    from pygments.lexers import get_lexer_by_name
+    from pygments.lexers import get_lexer_by_name, TextLexer
     from pygments.formatters import HtmlFormatter
 
     def pygments_highlight(text, lang, style):