Fix compatibility with pipenv >= 2022.10.9

[felixonmars]

Project._lockfile became a function instead of a property in pypa/pipenv@b3381ce, which is shipped in pipenv since version 2022.10.9.

Adding a simple compatibility fix here fixes it for me.

Fixes #25

[felixonmars]

Note that the tests are still failing because pipenv moved away from pipfile completely which breaks the import of pipfile in test_parsing.py.

I have tried to rewrite the test logic to use plette following what pipenv did, but it turns out to be more complex than I thought.