fix: handle the license with exception case, e.g., llvmlite

[joanise]

Purpose of This Pull Request

Please check the relevant option by placing an "X" inside the brackets:

• Documentation update
• Bug fix
• New feature
• Other (please explain):

Overview of Changes

As described in #137, licensecheck currently hits an unhandled exception when faced with a license with exception, such as is found in llvmlite.

The underlying issue is that not all tokens handled on line 121 of packageinfo.py have a key attribute.

My solution is to guard against this problem with getattr(x, "key", str(x)) which fixes the problem.

Related Issue

If this pull request addresses a specific issue, please mention it using the
format "Fixes #IssueNumber"

Fixes #137

Reviewer Focus

It's a pretty simple patch, so nothing specific.

Additional Notes

I wanted to provide unit testing with my PR, but I did not manage to get the existing suite to run correctly on either my Windows or Linux machine. I've added a minimal data file for testing, however:

licensecheck -r tests/data/issue_137.txt

raises an exception on the current master branch but outputs llvmlite │ APACHE-2.0 WITH LLVM-EXCEPTION;; BSD-2-CLAUSE as expected with this PR.