Add MANIFEST w/ LICENSE reference

[pmlandwehr]

Hey-lo,

We're finishing up the conda build of traceback2 for conda-forge and would like to throw in a link to the LICENSE file. This can't be done with an explicit MANIFEST identifying the license as part of the source distribution. Would y'all consider adding a MANIFEST to future releases?

[rbtcollins]

a MANIFEST.in can be used; setuptools rewrites MANIFEST on build, so adding a MANIFEST itself would be pointless.