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ERROR: Error while checking for conflicts.

See original GitHub issue

pip version: 21.0.1
Python version:3.6
Operating system: ubuntu-18.04 aarch64

pip install gevent==1.3.0
Collecting gevent==1.3.0
  Downloading gevent-1.3.0.tar.gz (4.6 MB)
     |████████████████████████████████| 4.6 MB 28 kB/s
Collecting greenlet>=0.4.13
  Downloading greenlet-1.0.0-cp36-cp36m-manylinux2014_aarch64.whl (156 kB)
     |████████████████████████████████| 156 kB 27 kB/s
Building wheels for collected packages: gevent
  Building wheel for gevent (setup.py) ... done
  Created wheel for gevent: filename=gevent-1.3.0-cp36-cp36m-linux_aarch64.whl size=2264513 sha256=a385ba45d6b9512bca9187c51358f7003c016d93d0473b7396bedf419fef2356
  Stored in directory: /root/.cache/pip/wheels/7f/bf/28/9a2b10c1cb1c8ce68a030b420ef16ee4e206d1652b0bffe5b5
Successfully built gevent
ERROR: Error while checking for conflicts. Please file an issue on pip's issue tracker: https://github.com/pypa/pip/issues/new
Traceback (most recent call last):
  File "/usr/local/lib/python3.6/dist-packages/pip/_internal/commands/install.py", line 514, in _determine_conflicts
    return check_install_conflicts(to_install)
  File "/usr/local/lib/python3.6/dist-packages/pip/_internal/operations/check.py", line 115, in check_install_conflicts
    package_set, should_ignore=lambda name: name not in whitelist
  File "/usr/local/lib/python3.6/dist-packages/pip/_internal/operations/check.py", line 81, in check_package_set
    missed = req.marker.evaluate()
  File "/usr/local/lib/python3.6/dist-packages/pip/_vendor/packaging/markers.py", line 336, in evaluate
    return _evaluate_markers(self._markers, current_environment)
  File "/usr/local/lib/python3.6/dist-packages/pip/_vendor/packaging/markers.py", line 252, in _evaluate_markers
    lhs_value = _get_env(environment, lhs.value)
  File "/usr/local/lib/python3.6/dist-packages/pip/_vendor/packaging/markers.py", line 233, in _get_env
    "{0!r} does not exist in evaluation environment.".format(name)
pip._vendor.packaging.markers.UndefinedEnvironmentName: 'extra' does not exist in evaluation environment.
Installing collected packages: greenlet, gevent
Successfully installed gevent-1.3.0 greenlet-1.0.0