Getting exception with Python 3.9 and 0.9.2

Expected Behavior

Should work with Python 3.9.

Actual Behavior

Getting exception:

Traceback (most recent call last):
  File "/usr/local/bin/pdoc", line 8, in <module>
    sys.exit(main())
  File "/usr/local/lib/python3.9/site-packages/pdoc/cli.py", line 572, in main
    recursive_write_files(module, ext='.html', **template_config)
  File "/usr/local/lib/python3.9/site-packages/pdoc/cli.py", line 349, in recursive_write_files
    recursive_write_files(submodule, ext=ext, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/pdoc/cli.py", line 344, in recursive_write_files
    f.write(m.html(**kwargs))
  File "/usr/local/lib/python3.9/site-packages/pdoc/__init__.py", line 832, in html
    html = _render_template('/html.mako', module=self, **kwargs)
  File "/usr/local/lib/python3.9/site-packages/pdoc/__init__.py", line 158, in _render_template
    return t.render(**config).strip()
  File "/usr/local/lib/python3.9/site-packages/mako/template.py", line 476, in render
    return runtime._render(self, self.callable_, args, data)
  File "/usr/local/lib/python3.9/site-packages/mako/runtime.py", line 878, in _render
    _render_context(
  File "/usr/local/lib/python3.9/site-packages/mako/runtime.py", line 920, in _render_context
    _exec_template(inherit, lclcontext, args=args, kwargs=kwargs)
  File "/usr/local/lib/python3.9/site-packages/mako/runtime.py", line 947, in _exec_template
    callable_(context, *args, **kwargs)
  File "_html_mako", line 143, in render_body
  File "_html_mako", line 49, in show_module
  File "_html_mako", line 428, in render_show_module
  File "/usr/local/lib/python3.9/site-packages/pdoc/__init__.py", line 1085, in subclasses
    for c in type.__subclasses__(self.obj))
TypeError: descriptor '__subclasses__' for 'type' objects doesn't apply to a 'types.GenericAlias' object

Steps to Reproduce

1. git clone https://github.com/dbrattli/Expression.git
2. cd expression
3. pdoc --html expression -o docs

Additional info

• pdoc version: 0.9.2

Start at pdoc/init.py:1085, where subclasses fails, and reproduce with the Expression clone using pdoc --html expression -o docs under Python 3.9. Trace the handling of the types.GenericAlias shown in the traceback and verify that the command completes without the exception.