Support buffer line range deletion to pass Vim's python3 tests

[janlazo]

Currently, neovim/neovim#37155 fails on the following code:

py3 del r[:]

pynvim cannot delete buffer line using the Range object.
Vim expects deletion for all buffer lines within that Range.

Traceback (most recent call last):
  File "<string>", line 1, in <module>
    import sys; sys.path = [p for p in sys.path if p != ""]; import neovim; neovim.start_host()
        ^^^^^^
AttributeError: __delitem__
stack traceback:
        [C]: at 0x7ff6794fcaa0

Unsure if __delitem__() https://github.com/neovim/pynvim/blob/master/pynvim/api/buffer.py#L130 needs a method overload for Range when compared to __setitem__() method overloads that deal with different inputs.

Unsure if this was already addressed in issues/PRs like #278 because that test will become N/A. I already commented test cases that fail on Vim but pass on Nvim (due to Range?).

[brianhuster]

I personally think if_pyth tests should be moved to pynvim repo. After all most of its development happen here anyway

[justinmk]

I personally think if_pyth tests should be moved to pynvim repo. After all most of its development happen here anyway

Need in both repos, since both can cause regressions. In this repo, we can fetch neovim/neovim and run only its if_pyth tests.

Implemented in:

• ci: also run upstream Nvim if_pyth tests #611