fix: make mistletoe.HTMLRenderer work again

Backwards compatibility was actually broken in #182 inadvertently -
it didn't cover the case when the old `HTMLRenderer` was imported directly
from the `mistletoe` package.

Note: The `# noqa: F401` comment is for the flake8 linter, it suppresses
the `imported but unused` rule.

mistletoe/__init__.py

@@ -8,6 +8,9 @@
 
 from mistletoe.block_token import Document
 from mistletoe.html_renderer import HtmlRenderer
+# import the old name for backwards compatibility:
+from mistletoe.html_renderer import HTMLRenderer  # noqa: F401
+
 
 def markdown(iterable, renderer=HtmlRenderer):
     """