Initial nix-build fails to build

[wykurz]

Building on nixos 21.11 from current master (8390088) I get:

  Skipping optional fixer: idioms
  Skipping optional fixer: set_literal
  Skipping optional fixer: ws_comma
  regex_3/_regex.c: In function ‘folded_char_at’:
  regex_3/_regex.c:9897:9: warning: variable ‘folded_len’ set but not used [-Wunused-but-set-variable]
   9897 |     int folded_len;
        |         ^~~~~~~~~~
    mypy/api.py:52: error: syntax error in type comment
    mypy/api.py:66: error: syntax error in type comment
    mypy/api.py:73: error: syntax error in type comment
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
Exception information:
Traceback (most recent call last):
  File "/nix/store/ss06knypz76mhki89wxvzbpk2vvcs97b-python3.9-pip-20.2.4/lib/python3.9/site-packages/pip/_internal/cli/base_command.py", line 228, in _main
    status = self.run(options, args)
  File "/nix/store/ss06knypz76mhki89wxvzbpk2vvcs97b-python3.9-pip-20.2.4/lib/python3.9/site-packages/pip/_internal/cli/req_command.py", line 182, in wrapper
    return func(self, options, args)
  File "/nix/store/ss06knypz76mhki89wxvzbpk2vvcs97b-python3.9-pip-20.2.4/lib/python3.9/site-packages/pip/_internal/commands/wheel.py", line 155, in run
    requirement_set = resolver.resolve(
  File "/nix/store/ss06knypz76mhki89wxvzbpk2vvcs97b-python3.9-pip-20.2.4/lib/python3.9/site-packages/pip/_internal/resolution/legacy/resolver.py", line 183, in resolve
    discovered_reqs.extend(self._resolve_one(requirement_set, req))
  File "/nix/store/ss06knypz76mhki89wxvzbpk2vvcs97b-python3.9-pip-20.2.4/lib/python3.9/site-packages/pip/_internal/resolution/legacy/resolver.py", line 388, in _resolve_one
    abstract_dist = self._get_abstract_dist_for(req_to_install)
  File "/nix/store/ss06knypz76mhki89wxvzbpk2vvcs97b-python3.9-pip-20.2.4/lib/python3.9/site-packages/pip/_internal/resolution/legacy/resolver.py", line 340, in _get_abstract_dist_for
    abstract_dist = self.preparer.prepare_linked_requirement(req)
  File "/nix/store/ss06knypz76mhki89wxvzbpk2vvcs97b-python3.9-pip-20.2.4/lib/python3.9/site-packages/pip/_internal/operations/prepare.py", line 482, in prepare_linked_requirement
    abstract_dist = _get_prepared_distribution(
  File "/nix/store/ss06knypz76mhki89wxvzbpk2vvcs97b-python3.9-pip-20.2.4/lib/python3.9/site-packages/pip/_internal/operations/prepare.py", line 91, in _get_prepared_distribution
    abstract_dist.prepare_distribution_metadata(finder, build_isolation)
  File "/nix/store/ss06knypz76mhki89wxvzbpk2vvcs97b-python3.9-pip-20.2.4/lib/python3.9/site-packages/pip/_internal/distributions/sdist.py", line 40, in prepare_distribution_metadata
    self.req.prepare_metadata()
  File "/nix/store/ss06knypz76mhki89wxvzbpk2vvcs97b-python3.9-pip-20.2.4/lib/python3.9/site-packages/pip/_internal/req/req_install.py", line 555, in prepare_metadata
    self.metadata_directory = self._generate_metadata()
  File "/nix/store/ss06knypz76mhki89wxvzbpk2vvcs97b-python3.9-pip-20.2.4/lib/python3.9/site-packages/pip/_internal/req/req_install.py", line 530, in _generate_metadata
    return generate_metadata_legacy(
  File "/nix/store/ss06knypz76mhki89wxvzbpk2vvcs97b-python3.9-pip-20.2.4/lib/python3.9/site-packages/pip/_internal/operations/build/metadata_legacy.py", line 70, in generate_metadata
    call_subprocess(
  File "/nix/store/ss06knypz76mhki89wxvzbpk2vvcs97b-python3.9-pip-20.2.4/lib/python3.9/site-packages/pip/_internal/utils/subprocess.py", line 242, in call_subprocess
    raise InstallationError(exc_msg)
pip._internal.exceptions.InstallationError: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.
Removed file:///build/mypy-0.770 from build tracker '/build/pip-req-tracker-7cypa4b8'
Removed build tracker: '/build/pip-req-tracker-7cypa4b8'
builder for '/nix/store/v52f7jqk64nscbcbsxs8bvf3ccpg0lzc-python3.9-mypy-0.770.drv' failed with exit code 1
cannot build derivation '/nix/store/34dnzi1x8qdx4z0b0p8szbk0lqf7xq9r-python3-3.9.6-env.drv': 1 dependencies couldn't be built
error: build of '/nix/store/34dnzi1x8qdx4z0b0p8szbk0lqf7xq9r-python3-3.9.6-env.drv' failed