Fix read of py27 pytables tz attribute, gh#26443
When created by python 2.7, the "tz" attribute will be created with CSET
H5T_CSET_ASCII instead of H5T_CSET_UTF8, therefore it is read as bytes
when string is expected.

- [ X] closes #26443
- [ X] tests added / passed
- [ X] passes `black pandas`
- [ X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ X] whatsnew entry

Thanks for the PR but not sure about this as it makes it murky as to what aspects of Py27 we still support. Without this its very clear to say nothing, so I'm maybe -1 on this as I don't think we want to start down the path of offering any Py27 legacy support going forward.

Let's see what others think 1.0 (I don't think it was a 0.25.x regression).