The release attrs==21.1.0 broke pytype check

[plamut]

Running the pytype check on master fails:

$ nox -s pytype

...
    return attr.evolve(self, *args, **kwargs)
  File "/home/peter/workspace/python-bigquery/.nox/pytype/lib/python3.8/site-packages/attr/_funcs.py", line 347, in evolve
    changes[init_name] = evolve(value, **changes[init_name])
TypeError: evolve() argument after ** must be a mapping, not NamedType
...

This happens with the most recent attrs release, namely 21.1.0. The error does not happen in a release before that (attrs==20.3.0).

A possible workaround is to temporarily force installing that version before running pytype.