Find pyproject from vim relative to current file (#1871)

Commit history before merge:

* Find pyproject from vim relative to current file
* Merge remote-tracking branch 'upstream/main' into find-pyproject-vim
* Finish and fix this patch (thanks Matt Wozniski!)

Both the existing code and the proposed code are broken.
The vim.eval() call (whether it's vim.eval("@%") or
vim.eval("fnamemodify(getcwd(), ':t')) returns a string, and it passes
that string to find_pyproject_toml, which expects a sequence of strings,
not a single string, and - since a string is a sequence of single
character strings - it gets turned into a list of ridiculous paths. I
tested with a file called foo.py, and added a print(path_srcs) into
find_project_root, which printed out:

[
  PosixPath('/home/matt/f'),
  PosixPath('/home/matt/o'),
  PosixPath('/home/matt/o'),
  PosixPath('/home/matt'),
  PosixPath('/home/matt/p'),
  PosixPath('/home/matt/y')
]

This does work for an unnamed buffer, too - we wind up calling
black.find_pyproject_toml(("",)), and that winds up prepending the
working directory to any relative paths, so "" just gets turned into
the current working directory.

Note that find_pyproject_toml needs to be passed a 1-tuple, not a
list, because it requires something hashable (thanks to
functools.lru_cache being used)

Co-authored-by: Matt Wozniski <mwozniski@bloomberg.net>
* I forgot the CHANGELOG entry ... again
* I'm really bad at dealing with merge conflicts sometimes
* Be more correct describing search behaviour

Co-authored-by: Austin Glaser <austin.glaser@spacex.com>
Co-authored-by: Richard Si <63936253+ichard26@users.noreply.github.com>
Co-authored-by: Matt Wozniski <mwozniski@bloomberg.net>