BUG fix _Unstacker int32 limit in dataframe sizes (pandas-dev#26314)
- [x] closes #26314 
- [ ] tests added / passed
- [x] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry



I have tested it with :
`df = pd.DataFrame(np.random.randint(low=0, high=1500000, size=(90000, 2)), columns=['a', 'b'])`
`df.set_index(['a', 'b']).unstack()`

I am not sure if I should add it as a test, it requires quite some memory.  I am also hesitant about where the test should be located in case it's added.