Migrate isort, mypy, and pytest config into pyproject.toml

[cclauss]

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The issue

What is the thing you want to fix? Is it associated with an issue on GitHub? Please mention it.

PEP621 encourages the Python community to migrate away from complex setup.cfg + setup.py files by putting config data in pyproject.toml. Let's keep moving in that direction. https://github.com/abravalheri/ini2toml

Always consider opening an issue first to describe your problem, so we can discuss what is the best way to amend it. Note that if you do not describe the goal of this change or link to a related issue, the maintainers may close the PR without further review.

If your pull request makes a non-insignificant change to Pipenv, such as the user interface or intended functionality, please file a PEEP.

https://github.com/pypa/pipenv/blob/master/peeps/PEEP-000.md

The fix

How does this pull request fix your problem? Did you consider any alternatives? Why is this the best solution, in your opinion?

The checklist

• Associated issue
• A news fragment in the news/ directory to describe this fix with the extension .bugfix.rst, .feature.rst, .behavior.rst, .doc.rst. .vendor.rst. or .trivial.rst (this will appear in the release changelog). Use semantic line breaks and name the file after the issue number or the PR #.