Support Python 3.8

[Néstor Salceda]

When running Mamba on Python 3.8 I get the following error:

$ pipenv run mamba                 

Traceback (most recent call last):
  File "/home/nestor/.local/share/virtualenvs/mamba-EOHBIPe7/bin/mamba", line 11, in <module>
    load_entry_point('mamba', 'console_scripts', 'mamba')()
  File "/home/nestor/Projects/oss/mamba/mamba/cli.py", line 18, in main
    runner.run()
  File "/home/nestor/Projects/oss/mamba/mamba/runners.py", line 29, in run
    modules = self.example_collector.modules()
  File "/home/nestor/Projects/oss/mamba/mamba/example_collector.py", line 25, in modules
    with self._load_module_from(path) as module:
  File "/usr/lib64/python3.8/contextlib.py", line 113, in __enter__
    return next(self.gen)
  File "/home/nestor/Projects/oss/mamba/mamba/example_collector.py", line 57, in _load_module_from
    yield self._module_from_ast(name, path)
  File "/home/nestor/Projects/oss/mamba/mamba/example_collector.py", line 74, in _module_from_ast
    code = compile(tree, path, 'exec')
TypeError: required field "posonlyargs" missing from arguments