TypeError in _visit_container_type caused by multi-level dict
Input
```py
CODES = {
    "KEY": [
        1,
        3,
    ],
}
```

error:
```py
Traceback (most recent call last):
  File "/usr/local/Caskroom/miniconda/base/lib/python3.8/runpy.py", line 194, in _run_module_as_main
    return _run_code(code, main_globals, None,
  File "/usr/local/Caskroom/miniconda/base/lib/python3.8/runpy.py", line 87, in _run_code
    exec(code, run_globals)
  File "/Users/john.vandenberg/python/py2many/py2many/__main__.py", line 3, in <module>
    main()
  File "/Users/john.vandenberg/python/py2many/py2many/cli.py", line 273, in main
    _process_once(settings, source, outdir)
  File "/Users/john.vandenberg/python/py2many/py2many/cli.py", line 209, in _process_once
    transpile(
  File "/Users/john.vandenberg/python/py2many/py2many/cli.py", line 82, in transpile
    code = transpiler.visit(tree) + "\n"
  File "/Users/john.vandenberg/python/py2many/py2many/clike.py", line 142, in visit
    return super().visit(node)
  File "/usr/local/Caskroom/miniconda/base/lib/python3.8/ast.py", line 363, in visit
    return visitor(node)
  File "/Users/john.vandenberg/python/py2many/pyrs/transpiler.py", line 404, in visit_Module
    buf += [self.visit(b) for b in node.body]
  File "/Users/john.vandenberg/python/py2many/pyrs/transpiler.py", line 404, in <listcomp>
    buf += [self.visit(b) for b in node.body]
  File "/Users/john.vandenberg/python/py2many/py2many/clike.py", line 142, in visit
    return super().visit(node)
  File "/usr/local/Caskroom/miniconda/base/lib/python3.8/ast.py", line 363, in visit
    return visitor(node)
  File "/Users/john.vandenberg/python/py2many/pyrs/transpiler.py", line 681, in visit_Assign
    typename = self._typename_from_annotation(node.value)
  File "/Users/john.vandenberg/python/py2many/py2many/clike.py", line 126, in _typename_from_annotation
    return self._visit_container_type(node.container_type)
  File "/Users/john.vandenberg/python/py2many/py2many/clike.py", line 101, in _visit_container_type
    index_type = ", ".join(index_type)
TypeError: sequence item 1: expected str instance, NoneType found
```