Probably incompatible with jinja2 >= 3.0

[igneus]

On Python 3.8 i pip-installed chant21, but it crashes on import

Python 3.8.10 (default, May  6 2021, 00:05:59) 
[GCC 10.2.1 20201203] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> import chant21
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/lib/python3.8/site-packages/chant21/__init__.py", line 16, in <module>
    from .gabc import *
  File "/usr/lib/python3.8/site-packages/chant21/gabc/__init__.py", line 2, in <module>
    from .converter import *
  File "/usr/lib/python3.8/site-packages/chant21/gabc/converter.py", line 10, in <module>
    from .. import chant
  File "/usr/lib/python3.8/site-packages/chant21/chant.py", line 26, in <module>
    from .html import toFile
  File "/usr/lib/python3.8/site-packages/chant21/html/__init__.py", line 42, in <module>
    WIDGET = TEMPLATE_ENV.get_template('widget.html')
  File "/usr/lib/python3.8/site-packages/jinja2/environment.py", line 997, in get_template
    return self._load_template(name, globals)
  File "/usr/lib/python3.8/site-packages/jinja2/environment.py", line 958, in _load_template
    template = self.loader.load(self, name, self.make_globals(globals))
  File "/usr/lib/python3.8/site-packages/jinja2/loaders.py", line 137, in load
    code = environment.compile(source, name, filename)
  File "/usr/lib/python3.8/site-packages/jinja2/environment.py", line 750, in compile
    source = self._generate(source, name, filename, defer_init=defer_init)
  File "/usr/lib/python3.8/site-packages/jinja2/environment.py", line 678, in _generate
    return generate(  # type: ignore
  File "/usr/lib/python3.8/site-packages/jinja2/compiler.py", line 117, in generate
    generator.visit(node)
  File "/usr/lib/python3.8/site-packages/jinja2/visitor.py", line 40, in visit
    return f(node, *args, **kwargs)
  File "/usr/lib/python3.8/site-packages/jinja2/compiler.py", line 888, in visit_Template
    self.blockvisit(node.body, frame)
  File "/usr/lib/python3.8/site-packages/jinja2/compiler.py", line 449, in blockvisit
    self.visit(node, frame)
  File "/usr/lib/python3.8/site-packages/jinja2/visitor.py", line 40, in visit
    return f(node, *args, **kwargs)
  File "/usr/lib/python3.8/site-packages/jinja2/compiler.py", line 1582, in visit_AssignBlock
    self.pop_assign_tracking(frame)
  File "/usr/lib/python3.8/site-packages/jinja2/compiler.py", line 813, in pop_assign_tracking
    ref = frame.symbols.ref(name)
  File "/usr/lib/python3.8/site-packages/jinja2/idtracking.py", line 80, in ref
    raise AssertionError(
AssertionError: Tried to resolve a name to a reference that was unknown to the frame ('_')

The error seems to be caused by

chant21/chant21/html/widget.html

Line 8 in 475689c

{%- for _ in range(5) -%}

Installed jinja2 version:

>>> import jinja2
>>> jinja2.__version__
'3.0.1'

Downgrading jinja2 to 2.11.3 (current result of pip install jinja2<3.0) fixes the issue. It seems that the current jinja2 version constraint in setup.py ("Jinja2>=2.11.1") is not completely accurate and the library is in fact incompatible with jinja2 >= 3.0.