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
