Co-authored-by: github-actions[bot] <github-actions[bot]@users.noreply.github.com>

Cross-reference stash keys start with `_` and preserve the rendered identifier length. One-character and two-character identifiers therefore share a keyspace of only 62 keys. Once all keys are present, `_gen_stash_key` retries random choices forever. This happens for example in mkdocstrings when rendering an attribute value that has 62 names in it or more. We increase the minimum key length by 1, to reach a keyspace of 3844, making it very unlikely to hit the limit again.

PR-336: #336

Issue-337: #337

When an object's file lives in a virtual environment inside the current working directory (for example a pydantic `BaseModel` subclass documented with `preload_modules: [pydantic]` and an in-project `.venv`), source blocks were labeled with the environment path, like `.venv/lib/python3.10/site-packages/pydantic/main.py`.

The path is relative to the current working directory, so it slipped past the `is_absolute()` check that normally keeps environment paths out of rendered docs.

Add a `source_location` filter that strips everything up to and including a `site-packages` directory from displayed source paths, and use it for every source block label: merged `__init__`, class, and function, in both themes. 

Centralizing the logic also fixes the label for single-file modules installed directly in `site-packages` (like `six.py`), whose package-relative path still contained the environment directory. Labels of objects belonging to the documented package itself are unchanged.

Issue-333: #333
PR-338: #338