rust-cpython: make PyLeakedRef operations relatively safe
This patch encapsulates the access to the leaked reference to make most
leaked-ref operations safe. The only exception is leaked_ref.map(). I
couldn't figure out how to allow arbitrary map operation safely over an
unsafe static reference. See the docstring and inline comment for details.
Now leak_immutable() can be safely implemented as the PyLeakedRef owns
its inner data.
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
| hgext/fsmonitor/pywatchman/
| contrib/python-zstandard/
| contrib/grey.py
'''
skip-string-normalization = true
quiet = true