The documentation had a `logo.svg` file sitting in the docs directory
but it was never wired up in `conf.py`, so Sphinx never rendered it. The
logo itself was also a generic monochrome lock sketch that didn't
reflect the project's identity.

This replaces it with a proper branded logo that uses Python's official
blue (#3776AB) and yellow (#FFD43B) in a diagonal split gradient, with a
clean white lock icon on top. The `html_logo` setting is now configured
in `conf.py` so the logo actually appears in the Furo sidebar.

---------

Signed-off-by: Bernát Gábor <bgabor8@bloomberg.net>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

<!--pre-commit.ci start-->
updates:
- [github.com/python-jsonschema/check-jsonschema: 0.36.2 →
0.37.0](python-jsonschema/check-jsonschema@0.36.2...0.37.0)
- [github.com/tox-dev/tox-toml-fmt: v1.7.2 →
v1.8.0](tox-dev/tox-toml-fmt@v1.7.2...v1.8.0)
- [github.com/astral-sh/ruff-pre-commit: v0.15.2 →
v0.15.4](astral-sh/ruff-pre-commit@v0.15.2...v0.15.4)
<!--pre-commit.ci end-->

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Version 3.25.0 introduced a regression on Windows where lock files were
no longer cleaned up after release, leaving orphaned `.lock` files on
disk. 🗑️ Users upgrading from 3.18.0 reported that their single-threaded
applications now leave persistent lock files, breaking the expected
behavior and diverging from how Unix platforms work (where lock files
are cleaned up).

The regression came from PR #484, which removed the `unlink()` call to
fix threading race conditions. While that fix correctly addressed
multi-threaded scenarios where Windows cannot delete files with open
handles, it sacrificed cleanup for the common single-threaded use case
where deletion would succeed.

This fix restores opportunistic cleanup by attempting to unlink the lock
file after closing it, with errors suppressed via `suppress(OSError)`. ✨
In single-threaded scenarios, the file deletes successfully and users
get the expected behavior. In multi-threaded scenarios where other
threads hold handles, the deletion fails silently and the lock file
persists, preserving the thread-safety guarantees from PR #484.

The test suite is updated to remove the Windows skip condition from
`test_lock_file_removed_after_release`, as Windows now supports cleanup
in typical usage patterns.

Closes #509

<!--pre-commit.ci start-->
updates:
- [github.com/codespell-project/codespell: v2.4.1 →
v2.4.2](codespell-project/codespell@v2.4.1...v2.4.2)
- [github.com/tox-dev/tox-toml-fmt: v1.8.0 →
v1.9.1](tox-dev/tox-toml-fmt@v1.8.0...v1.9.1)
- [github.com/astral-sh/ruff-pre-commit: v0.15.4 →
v0.15.5](astral-sh/ruff-pre-commit@v0.15.4...v0.15.5)
<!--pre-commit.ci end-->

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>