Support isolated builds for toxfile

[gnikonorov]

Tox supports isolated builds and we should take advantage of this.

Supporting this feature would require the creation of a pyproject.toml to inform tox how to perform the build. The one in pytest may be a good starting point.

Raised from description in https://github.com/pytest-dev/pytest-rerunfailures/pull/117

[sallner]

As I cannot quickly get all the pros and cons about the isolated builds in my head, could you please provide some more information on this?

[graingert]

@sallner isolated builds should speed up the tests a bit as it will only build the dist once - also it makes the tests more realistic as you are testing on an artefact even close to the end product shipped to pypi

[sallner]

I am not yet convinced about adding another configuration file, but let's give it a try. Are there any caveats concerning coverage?

[graingert]

pytest and tox will both read from pyproject.toml