shelve: open patch using new shelf class instead of open()
For some reason the existing code didn't use `shelvedfile().opener()`
so here we migrate to `shelf().open_patch()` from the `open()` system
call instead.
Differential Revision: https://phab.mercurial-scm.org/D9706
[tool.black]
line-length = 80
exclude = '''
build/
| wheelhouse/
| dist/
| packages/
| \.hg/
| \.mypy_cache/
| \.venv/
| mercurial/thirdparty/
'''
skip-string-normalization = true
quiet = true