Broken on Python 3 with a custom template

[dstufft]

Trying to use a custom template on Python 3.x results in:

Loading template...
Finding news fragments...
Rendering news fragments...
Traceback (most recent call last):
  File "/Users/dstufft/.virtualenvs/tmp-46f4170e7667d3a/bin/towncrier", line 11, in <module>
    sys.exit(_main())
  File "/Users/dstufft/.virtualenvs/tmp-46f4170e7667d3a/lib/python3.6/site-packages/click/core.py", line 722, in __call__
    return self.main(*args, **kwargs)
  File "/Users/dstufft/.virtualenvs/tmp-46f4170e7667d3a/lib/python3.6/site-packages/click/core.py", line 697, in main
    rv = self.invoke(ctx)
  File "/Users/dstufft/.virtualenvs/tmp-46f4170e7667d3a/lib/python3.6/site-packages/click/core.py", line 895, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/dstufft/.virtualenvs/tmp-46f4170e7667d3a/lib/python3.6/site-packages/click/core.py", line 535, in invoke
    return callback(*args, **kwargs)
  File "/Users/dstufft/.virtualenvs/tmp-46f4170e7667d3a/lib/python3.6/site-packages/towncrier/__init__.py", line 36, in _main
    return __main(draft, directory, project_version, project_date)
  File "/Users/dstufft/.virtualenvs/tmp-46f4170e7667d3a/lib/python3.6/site-packages/towncrier/__init__.py", line 74, in __main
    rendered = render_fragments(template, fragments, definitions)
  File "/Users/dstufft/.virtualenvs/tmp-46f4170e7667d3a/lib/python3.6/site-packages/towncrier/_builder.py", line 101, in render_fragments
    jinja_template = Template(template, trim_blocks=True)
  File "/Users/dstufft/.virtualenvs/tmp-46f4170e7667d3a/lib/python3.6/site-packages/jinja2/environment.py", line 945, in __new__
    return env.from_string(source, template_class=cls)
  File "/Users/dstufft/.virtualenvs/tmp-46f4170e7667d3a/lib/python3.6/site-packages/jinja2/environment.py", line 880, in from_string
    return cls.from_code(self, self.compile(source), globals, None)
  File "/Users/dstufft/.virtualenvs/tmp-46f4170e7667d3a/lib/python3.6/site-packages/jinja2/environment.py", line 581, in compile
    defer_init=defer_init)
  File "/Users/dstufft/.virtualenvs/tmp-46f4170e7667d3a/lib/python3.6/site-packages/jinja2/environment.py", line 543, in _generate
    optimized=self.optimized)
  File "/Users/dstufft/.virtualenvs/tmp-46f4170e7667d3a/lib/python3.6/site-packages/jinja2/compiler.py", line 78, in generate
    raise TypeError('Can\'t compile non template nodes')
TypeError: Can't compile non template nodes

It appears this is because the template is being read as bytes instead of a string.