Enable ruff's flake8-pytest-style (PT) rules

[seisman]

Enable ruff's flake8-pytest-style (PT) rules.

Currently, there are 6 rules that have violations:

ruff check pygmt doc/conf.py examples --extend-select=PT --statistics
164	PT023	[*] Use `@pytest.mark.mpl_image_compare()` over `@pytest.mark.mpl_image_compare`
  4	PT012	[ ] `pytest.raises()` block should contain a single simple statement
  3	PT004	[ ] Fixture `fixture_mock_x2sys_home` does not return anything, add leading underscore
  3	PT006	[*] Wrong name(s) type in `@pytest.mark.parametrize`, expected `tuple`
  2	PT017	[ ] Found assertion on exception `error` in `except` block, use `pytest.raises()` instead
  1	PT001	[*] Use `@pytest.fixture()` over `@pytest.fixture`

The first column is the number of violations for each rule.

Please check the ruff documentation for the explanations of each rule. I think I agree with all the rules except PT023. There are 164 violations and I'm OK with @pytest.mark.mpl_image_compare, so prefer to ignore this rule.