Clean up stale docstrings, history comments, and leftover CI/test cache files

These are small, and each item is a deletion or a one-line docstring fix. I left them out of #74 so that PR stays docs-only.

Docstrings and comments that are wrong or narrate history

• gtfparse/read_gtf.py:279: read_gtf says it returns "a dictionary mapping column names to sequences of values". It returns a Polars DataFrame by default.
• gtfparse/read_gtf.py:183: parse_gtf_and_expand_attributes documents a chunksize parameter it doesn't take.
• gtfparse/read_gtf.py:419-426: an unrecognized result_type silently returns None. It should raise ValueError.
• gtfparse/write_gtf.py:61-63: the docstring refers to "the naive if value: check in earlier drafts".
• gtfparse/attribute_parsing.py:55-56: the comment explains intern(str(...)) (:81) as a Python 2 unicode workaround. On Python 3 the str() call does nothing.
• tests/test_gencode_gtf.py:17,102,160: section dividers labeled with issue numbers (#63, #64). tests/test_expand_attribute_column_false.py:1-4 describes the old NameError instead of the behavior under test.
• pyproject.toml:74-80: ruff ignore comments like "out of scope for config PR".

Stale infrastructure

• test.sh:3-4 points to ~/code/trufflepig/test.sh for its rationale. That path exists only on a local machine.
• .github/workflows/tests.yml:4-8: the TODO block (cache ~/.cache/pyensembl, conda binaries, add a tests badge, update coveralls) doesn't apply. The badge is already in the README and Coveralls already uses coverallsapp/github-action@v2.2.3.
• .hypothesis/unicode_data/13.0.0/charmap.json.gz and tests/.hypothesis/... are committed Hypothesis caches. Nothing imports Hypothesis. Delete them and add .hypothesis/ to .gitignore.

format.sh (apply formatting), lint.sh (ruff check plus format check), and lint-and-test.sh (lint, then test) don't overlap, so I'd keep all three.

Start with the listed locations in gtfparse/read_gtf.py, gtfparse/write_gtf.py, gtfparse/attribute_parsing.py, the named test files, pyproject.toml, test.sh, and .github/workflows/tests.yml; run lint-and-test.sh before changes. Done means the stale docstrings, comments, TODOs, local path, and committed Hypothesis caches are removed, the invalid result_type raises ValueError, .hypothesis/ is ignored, and the three helper scripts remain.