Fix some broken tests (and improve developer experience)

[n-takumasa]

Ruff claims some errors, but fixes are not included in this PR.

Additional information:
The number of stencil elements differs between windows and macos.
https://github.com/n-takumasa/findiff/actions/runs/12427590734/job/34697664219

tests/test_bugs.py:132: in assert_dict_almost_equal
    self.assertEqual(len(actual), len(expected))
E   AssertionError: 4 != 5
        actual     = {(-2, 0): 0.08333333333333333,
 (-1, 0): -0.6666666666666666,
 (1, 0): 0.6666666666666666,
 (2, 0): -0.08333333333333333}
        expected   = {(np.int64(-2), np.int64(0)): np.float64(0.08333333333333334),
 (np.int64(-1), np.int64(0)): np.float64(-0.6666666666666667),
 (np.int64(0), np.int64(0)): np.float64(2.379049338482478e-16),
 (np.int64(1), np.int64(0)): np.float64(0.6666666666666665),
 (np.int64(2), np.int64(0)): np.float64(-0.08333333333333333)}
        places     = 7
        self       = <tests.test_bugs.TestOldBugs testMethod=test_accuracy_should_be_passed_down_to_stencil>
=========================== short test summary info ============================
FAILED tests/test_bugs.py::TestOldBugs::test_accuracy_should_be_passed_down_to_stencil - AssertionError: 4 != 5
=================== 1 failed, 157 passed, 3 skipped in 1.75s ===================
Error: Process completed with exit code 1.

[maroba]

Why include "tests" in source_pkgs? In the workflow, the command that is called for coverage is:

pytest --cov=findiff --cov-report=html tests

[n-takumasa]

I consider it a best practice because unexecuted lines in a test file smell like a bug.
This can detect function name collisions. (test_calc_accuracy_from_offsets_symbolic)

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