[3.12] gh-107396: tarfiles: set self.exception before _init_read_gz() (GH-107485...

[3.12] gh-107396: tarfiles: set self.exception before _init_read_gz() (GH-107485) (#108207)

gh-107396: tarfiles: set self.exception before _init_read_gz() (GH-107485)

In the stack call of: _init_read_gz()
```
_read, tarfile.py:548
read, tarfile.py:526
_init_read_gz, tarfile.py:491
```
a try;except exists that uses `self.exception`, so it needs to be set before
calling _init_read_gz().
(cherry picked from commit 37135d25e269ede92bc7da363bebfa574782e59a)

Co-authored-by: balmeida-nokia <83089745+balmeida-nokia@users.noreply.github.com>