Use pipenv to build the tox test environments.

[marcofavorito]

Proposed changes

This PR makes tox to use pipenv install --dev to build the test environment from Pipfile.lock.

That would help in running the tests with the fixed dependencies version numbers declared in Pipfile.lock.

Fixes

None.

Types of changes

What types of changes does your code introduce to agents-tac?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING doc
• I am making a pull request against the develop branch (left side). Also you should start your branch off our develop.
• Lint and unit tests pass locally with my changes
• I have added tests that prove my fix is effective or that my feature works

Further comments

None.