Restore compatibility with mistune==0.8.4

The latest release version of Lektor is 3.3.4.  It does not include
the support for `mistune>=2.0` that has made it into the `master`
branch of Lektor, thus it pins `mistune<2`.

This commit restores compatibility with older versions of
mistune.  (That compatibility was broken by 3fcd062.)

packages/markdown-link-classes/lektor_markdown_link_classes.py

@@ -1,6 +1,7 @@
 import re
 from lektor.pluginsystem import Plugin
 from markupsafe import escape
+from mistune import __version__ as mistune_version
 
 
 _class_re = re.compile(r'\s+:([a-zA-Z0-9_-]+)')
@@ -15,21 +16,31 @@ def split_classes(text):
     return text, classes
 
 
+def render_link(link, text, title=None):
+    text, classes = split_classes(text)
+    if link.startswith('javascript:'):
+        link = ''
+    attr = [f'href="{escape(link)}"']
+    if title:
+        attr.append(f'title="{escape(title)}"')
+    if classes:
+        attr.append(f'class="{" ".join(map(escape, classes))}"')
+    return f"<a {' '.join(attr)}>{text if text else link}</a>"
+
+
+if mistune_version.startswith("0."):
+    class LinkClassesMixin(object):
+        def link(renderer, link, title, text):
+            return render_link(link, text, title)
+else:
+    class LinkClassesMixin(object):
+        def link(renderer, link, text, title):
+            return render_link(link, text, title)
+
+
 class MarkdownLinkClassesPlugin(Plugin):
     name = 'Markdown Link Classes'
     description = 'Adds the ability to add classes to links.'
 
     def on_markdown_config(self, config, **extra):
-        class LinkClassesMixin(object):
-            def link(renderer, link, text, title):
-                text, classes = split_classes(text)
-                if link.startswith('javascript:'):
-                    link = ''
-                attr = ['href="%s"' % escape(link)]
-                if title:
-                    attr.append('title="%s"' % escape(title))
-                if classes:
-                    attr.append('class="%s"' % ' '.join(
-                        escape(x) for x in classes))
-                return '<a %s>%s</a>' % (' '.join(attr), text)
         config.renderer_mixins.append(LinkClassesMixin)