Fix out of bounds NvimError: Add iter to VimBuffer

[idbrii]

Fix #1041: iterating over snip.buffer fails in neovim.

Use iter() to defer iteration of a buffer to the underlying object.

Test

• Run this code in neovim and vim: import vim import UltiSnips.vim_helper as helper

    def dump(b):
        for line in b:
            print(line.upper())

    # This works fine in both nvim and vim.
    dump(vim.current.buffer)

    # In nvim, the iteration previously failed with "pynvim.api.common.NvimError: Index out of bounds"
    dump(helper.VimBuffer())

[idbrii]

Draft because I don't know how to write or run tests. (I'm on Windows.)

[idbrii]

I couldn't figure out the tests, so I removed it from my PR. Here's my proposed test (just guessing how to specify the contents of the buffer):

class PythonCode_IterateBuffer(_VimTest):
    snippets = ("test", """Start `!p
        snip.rv = ""
        for line in snip.buffer:
            snip.rv += "x"
        ` End""")
    buffer = """a
        b
        c
        d
        """
    keys = "test" + EX
    wanted = "Start xxxx End"

The change is simple enough that it may not warrant the test.