rust-cpython: make PySharedRef::try_borrow_mut() return BorrowMutError
As I said, it shouldn't be an error of Python layer, but is something like
a coding error. Returning BorrowMutError makes more sense.
There's a weird hack to propagate the borrow-by-leaked state to RefCell
to obtain BorrowMutError. If we don't like it, maybe we can add our own
BorrowMutError.
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true