Allow pluggy 1.0.0

[The-Compiler]

Recently, pluggy 1.0.0 was released, but diff_cover depends on ^0.13.1, i.e. < 1.0.0.

There were relatively few backwards-incompatible changes, so I'd expect it to (hopefully) be a drop-in replacement, like it was e.g. for pytest: pytest-dev/pytest#9040

[kasium]

Thanks for the heads up. I opened a PR and waiting for @Bachmann1234 for approval/feedback