Without this change, the Makefile fails when using Python 3.
```
mkdir -p _includes
bibble bib/pubs.bib bib/publications.tmpl > _includes/pubs.html
Traceback (most recent call last):
File "/usr/local/bin/bibble", line 8, in <module>
sys.exit(main())
File "/usr/local/lib/python3.8/site-packages/click/core.py", line 829, in __call__
return self.main(*args, **kwargs)
File "/usr/local/lib/python3.8/site-packages/click/core.py", line 782, in main
rv = self.invoke(ctx)
File "/usr/local/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
return ctx.invoke(self.callback, **ctx.params)
File "/usr/local/lib/python3.8/site-packages/click/core.py", line 610, in invoke
return callback(*args, **kwargs)
File "/usr/local/lib/python3.8/site-packages/bibble/main.py", line 183, in main
out = tmpl.render(entries=bib_sorted)
File "/usr/local/lib/python3.8/site-packages/jinja2/environment.py", line 1090, in render
self.environment.handle_exception()
File "/usr/local/lib/python3.8/site-packages/jinja2/environment.py", line 832, in handle_exception
reraise(*rewrite_traceback_stack(source=source))
File "/usr/local/lib/python3.8/site-packages/jinja2/_compat.py", line 28, in reraise
raise value.with_traceback(tb)
File "<template>", line 37, in top-level template code
File "/usr/local/lib/python3.8/site-packages/jinja2/sandbox.py", line 460, in call
if not __self.is_safe_callable(__obj):
File "/usr/local/lib/python3.8/site-packages/jinja2/sandbox.py", line 360, in is_safe_callable
getattr(obj, "unsafe_callable", False) or getattr(obj, "alters_data", False)
jinja2.exceptions.UndefinedError: 'dict object' has no attribute 'iteritems'
make: *** [_includes/pubs.html] Error 1
```
Josh Pollock
Adrian Sampson