Tox env with no dependencies

[elacuesta]

Fixes #10

Two problems with this:

• ~Merging reports does not seem to be working, coverage report shows a 13.9% decrease that is not actually accurate.~
• attrs is a dependency of pytest, so it gets installed. I tried to mock it but it's tricky, we need to prevent all imports from within itemadapter but not those within pytest; not sure it's worth it.