BUG: binary operator method alignment with integer level (GH9463)

[jorisvandenbossche]

Closes #9463

@jreback also related to #6682 (I removed 'join compat' introduced in that PR, as this worked only for level names, and passing level=level works for both level names and integer level numbers)

[jreback]

this wasn't being hit anywhere?

[jorisvandenbossche]

As far as I understand the code, this was hit, and this fixed the issue in the PR I linked, but it only fixed it for the case when you provide real level names for level (in this case, other is given the same name, and then Index.join will automatically join on the common level name.
When passing integer number to level, setting this number as the level name does not ensure that Index.join can join it correctly, as there are no common level names.

Another option is to set here the correct level name from self that corresponds to the provided level number, but as passing level=level to join also works (for both cases) and is simpler, I took that option.

[jreback]

ok, your soln seems right/reasonable.

[jorisvandenbossche]

ok! still have to add a release note