[TST] Bump pytest>=8 and resolve deprecations

[drewejohnson]

Closes #436 by bumping pytest to >=8. In doing that, we need to remove our reliance on a deprecated feature, namely patterns like

with pytest.raises(None) as record:
    pass
assert len(record) == 0

when checking no warnings are raised. Now, we use the recwarn fixture like

def test_sample(recwarn):
    assert len(recwarn) == 0

Constructing a RegularPolygon has slightly changed in Matplotlib 3.8 to only accept the xy location and number of edges as positional arguments. So now the radius and orientation arguments are passed as keyword arguments

• PR fits the project scope
• Code to be committed is DRY and matches the code style of the project
• New and/or changed features are well documented and have adequate testing

[DanKotlyar]

@drewejohnson should I merge?