fix bug of overflow validaion in 'reshape'

[chjinche]

• closes Pivot / unstack on large data frame does not work int32 overflow #26314
• tests added / passed
• passes black pandas
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[pep8speaks]

Hello @chjinche! Thanks for opening this PR. We checked the lines you've touched for PEP 8 issues, and found:

• In the file pandas/core/reshape/reshape.py:

Line 119:89: E501 line too long (93 > 88 characters)

[simonjayhawkins]

Thanks @chjinche for the PR

can you add a release note and a test (for the test I think can use #26314 (comment))

[simonjayhawkins]

@chjinche closing as stale. ping to reopen if you want to continue.