Trac: Mastertickets throws template error

[nielx]

After the update to Trac 1.6.0, the mastertickets plugin throws the following error sometimes:

2024-03-18 21:34:31,739 Trac[chrome] ERROR: Jinja2 TypeError error while rendering XML/HTML template 
Traceback (most recent call last):
  File "/opt/pypy/lib/pypy3.10/site-packages/trac/web/chrome.py", line 1569, in iterable_content
    for chunk in stream:
  File "/opt/pypy/lib/pypy3.10/site-packages/jinja2/environment.py", line 1662, in __next__
    return self._next()  # type: ignore
  File "/opt/pypy/lib/pypy3.10/site-packages/jinja2/environment.py", line 1639, in _buffered_generator
    c = next(self._gen)
  File "/opt/pypy/lib/pypy3.10/site-packages/jinja2/environment.py", line 1354, in generate
    yield self.environment.handle_exception()
  File "/opt/pypy/lib/pypy3.10/site-packages/jinja2/environment.py", line 936, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "/root/.cache/Python-Eggs/TracMasterTickets-5.0.0.dev0-py3.10.egg-tmp/mastertickets/templates/depgraph.html", line 1, in top-level template code
    # extends 'layout.html'
  File "/opt/pypy/lib/pypy3.10/site-packages/trac/templates/layout.html", line 12, in top-level template code
    # import "macros.html" as jmacros with context
  File "/opt/pypy/lib/pypy3.10/site-packages/trac/templates/theme.html", line 22, in top-level template code
    # block body
  File "/opt/pypy/lib/pypy3.10/site-packages/trac/templates/theme.html", line 130, in block 'body'
    # block content
  File "/root/.cache/Python-Eggs/TracMasterTickets-5.0.0.dev0-py3.10.egg-tmp/mastertickets/templates/depgraph.html", line 40, in block 'content'
    ${graph_render('cmapx').decode("utf-8") | safe}
  File "/var/trac/dev.haiku-os.org/plugins/TracMasterTickets-5.0.0.dev0-py3.10.egg/mastertickets/graphviz.py", line 143, in render
    out, error = p.communicate(to_unicode(self).encode('utf8'))
  File "/opt/pypy/lib/pypy3.10/site-packages/trac/util/text.py", line 135, in to_unicode
    return str(text)
  File "/var/trac/dev.haiku-os.org/plugins/TracMasterTickets-5.0.0.dev0-py3.10.egg/mastertickets/graphviz.py", line 133, in __str__
    lines.append(u'\t%s;' % obj)
TypeError: __str__ returned non-string (type 'int')

To be investigated and fixed.