BUG Decode to UTF-8 the dtype string read from a hdf file
Fixes GH31750

The dtype value wasn't being decoded to `UTF-8` when reading a DataFrame
from a hdf file. This was a problem when reading a hdf that was
created from python 2 with a fixed format as the dtype was being read as `b'datetime'`
instead of `datetime`, which caused `HDFStore` to read the data as
`int64` instead of coercing it to the correct `datetime64` dtype.

- [x] closes #31750
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

thanks thanks folks @jreback to get this fix backported to 1.0.2, do I need to open a second PR targeting that branch, or is it going to get picked up automatically by that backport bot? we don’t backport regular big fixes
they come out in the next major release  we don’t backport regular big fixes
they come out in the next major release 