[BUG] test_placement_table_box_elements permanently mutates the shared box.ASCII singleton

• I've checked docs and closed issues for possible solutions.
• I can't find my issue in the FAQ.

Describe the bug

tests/test_table.py::test_placement_table_box_elements builds a Table with the module-level box.ASCII singleton and then rewrites that singleton in place (table.box.__dict__.update(...), tests/test_table.py line 314) without restoring it. Any test that renders with box.ASCII afterwards sees the replacement letters instead of box-drawing characters:

python -m pytest tests/test_table.py::test_placement_table_box_elements tests/test_box.py::test_get_row -p no:randomly

def test_get_row():
    row = ASCII.get_row(widths=[1, 2, 3], level="row")
>   assert row == "|-+--+---|"
E   AssertionError: assert 'ijkjjkjjjl' == '|-+--+---|'

The suite only stays green because the natural file order happens to run test_box.py before test_table.py. Under pytest-randomly (which the test environment installs) the polluter runs first on roughly half the seeds: in a controlled comparison of 48 shuffled runs against 48 with the plugin disabled, the shuffled runs passed 52% and the natural order 100%, with hash seed, timezone, locale and allocator varied as controls that all stayed quiet.

Proposed fix

Give the table its own copy before mutating, for example box=copy.deepcopy(box.ASCII) in the Table constructed by that test, so the shared singleton stays pristine. Happy to send a PR if that direction works for you.

Platform

Reproduced on macOS (arm64), Python 3.13, pytest 9.1.1, pytest-randomly 5.0.0, at master 9d8f9a3. The failure depends only on test order, not on the platform.

Investigated with the help of Claude (Anthropic's Claude Code).