Upgrade pytest to version 6

[mondeja]

What does this changes

The unique change affected for 6 version is the mandatory use of keyword arguments in fixtures decorators, proposed in this issue.

What was wrong

Running the test suite with pytest 6 raises next warning error:

Passing arguments to pytest.fixture() as positional arguments is deprecated - pass them as a keyword argument instead

How this fixes it

Use scope='class' in tests/providers/test_barcode.py fixture.

[fcurella]

Thank you!