lit: Use a License classifier that pypi will accept

Summary:
'OSI Approved :: Apache-2.0 with LLVM exception' is not a valid
classifier.  'OSI Approved :: Apache Software License' is the closest
fit for the new license, so we've decided to use this one.

The classifiers seem to only be used for searching on the pypi website,
so this does not actually change the license of the code.
We still pass 'Apache-2.0 with LLVM exception' as the license to setup(),
and this appears alongside the classifier on the pypi webpage for lit.

Reviewers: chandlerc, ddunbar, joerg

Reviewed By: joerg

Subscribers: delcypher, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D65762

llvm-svn: 368315