`AttributeError: 'NoneType' object has no attribute 'iter_requirement'`

gistfile1.txt

**What did you want to do?**

I attempted to run `pip install -q -U seaborn scikit-learn model-card-toolkit` on pip version 20.3.1.

**Output**

```
ERROR: Exception:
Traceback (most recent call last):
  File "/tmpfs/src/tf_docs_env/lib/python3.6/site-packages/pip/_internal/cli/base_command.py", line 224, in _main
    status = self.run(options, args)
  File "/tmpfs/src/tf_docs_env/lib/python3.6/site-packages/pip/_internal/cli/req_command.py", line 180, in wrapper
    return func(self, options, args)
  File "/tmpfs/src/tf_docs_env/lib/python3.6/site-packages/pip/_internal/commands/install.py", line 321, in run
    reqs, check_supported_wheels=not options.target_dir
  File "/tmpfs/src/tf_docs_env/lib/python3.6/site-packages/pip/_internal/resolution/resolvelib/resolver.py", line 122, in resolve
    requirements, max_rounds=try_to_avoid_resolution_too_deep,
  File "/tmpfs/src/tf_docs_env/lib/python3.6/site-packages/pip/_vendor/resolvelib/resolvers.py", line 445, in resolve
    state = resolution.resolve(requirements, max_rounds=max_rounds)
  File "/tmpfs/src/tf_docs_env/lib/python3.6/site-packages/pip/_vendor/resolvelib/resolvers.py", line 325, in resolve
    for item in self.state.criteria.items()
  File "/tmpfs/src/tf_docs_env/lib/python3.6/site-packages/pip/_vendor/resolvelib/resolvers.py", line 326, in <listcomp>
    if not self._is_current_pin_satisfying(*item)
  File "/tmpfs/src/tf_docs_env/lib/python3.6/site-packages/pip/_vendor/resolvelib/resolvers.py", line 193, in _is_current_pin_satisfying
    for r in criterion.iter_requirement()
AttributeError: 'NoneType' object has no attribute 'iter_requirement'
```

**Additional information**

My `pip freeze`: https://gist.github.com/shuklak13/f68f9be33724abf3317d1829c17deb5c

My `pipdeptree`: https://gist.github.com/shuklak13/5c1c7b40714e782ff46ff250261dd55c