Upgrade to Python 3.9

I also needed to patch my Jinja2 filters to not use a deprecated method
of declaring themselves, since the upgrade pulled in a newer minor
version of Jinja2.

The upgrade also tried to pull in Django 4, which many of the plugins
I'm using can't cope with yet, so it needed to be convinced not to do
that.

lemoncurry/jinja2/bleach.py

@@ -1,6 +1,7 @@
 from bleach.sanitizer import Cleaner, ALLOWED_TAGS
 from bleach.linkifier import LinkifyFilter
-from jinja2 import evalcontextfilter, Markup
+from jinja2 import pass_eval_context
+from markupsafe import Markup
 
 TAGS = ['cite', 'code', 'details', 'p', 'pre', 'img', 'span', 'summary']
 TAGS.extend(ALLOWED_TAGS)
@@ -14,7 +15,7 @@ ATTRIBUTES = {
 cleaner = Cleaner(tags=TAGS, attributes=ATTRIBUTES, filters=(LinkifyFilter,))
 
 
-@evalcontextfilter
+@pass_eval_context
 def bleach(ctx, html):
     res = cleaner.clean(html)
     if ctx.autoescape:

lemoncurry/jinja2/markdown.py

@@ -1,4 +1,4 @@
-from jinja2 import evalcontextfilter
+from jinja2 import pass_eval_context
 from markdown import Markdown
 
 from .bleach import bleach
@@ -11,6 +11,6 @@ md = Markdown(extensions=(
 ))
 
 
-@evalcontextfilter
+@pass_eval_context
 def markdown(ctx, source):
     return bleach(ctx, md.reset().convert(source))