[3006.x] fix yumpkg for Python <3.8

[hurzhurz]

What does this PR do?

The yumpkg module doesn't not work anymore with Ptyhon versions older than 3.8 because of the usage of the assignment/named expression.
This is a trivial change to fix it.

What issues does this PR fix or reference?

Fixes #67091

[dwoz]

We do not support python lower than 3.10 in 3006.x.

[hurzhurz]

We do not support python lower than 3.10 in 3006.x.

Yes, I understand that it is not officially supported for older python versions.
But it helps a lot that it still works (at least for the most part). Especially for salt-ssh.
And thankfully also the requirements files are kept up to date for python versions down to 3.7.

I guess this tiny change wouldn't harm to keep it working, like it did before 3006.9 ?
And as far as I can tell, this is also the only occurrence of the assignment expression := in 3006 so far.

[dwoz]

I'm okay with this going into 3006.x. However, let's not merge change this forward. At least, not to master. There are many of these operators in master branch at this point.