pipenv: fix lockfile

with pipenv 2022.11.30, `null` marker could be generated, which would
fail other tools; this issue has been tracked under
pypa/pipenv#5529 and merged; current lockfile
generated with 2023.2.18

```
...
  File
"/builds/webgroup/fatcat-scholar/.venv/lib/python3.8/site-packages/packaging/_tokenizer.py",
line 163, in raise_syntax_error
    raise ParserSyntaxError(
pip._vendor.packaging._tokenizer.ParserSyntaxError: Expected a marker
variable or quoted string
    None
...
```