add packaging to the dependencies

[keewis]

Since a few people reported trouble with the setuptools vendoring mechanism (mainly because the ArchLinux package of setuptools does not include the vendoring part), this makes pint depend directly on packaging. We also depend on setuptools which in turn depends on packaging, but this way the dependency graph is more explicit.

We can revert this once we require numpy >= 1.16 and are able to remove the version check.

• Closes Problem with Setuptools #1008 (@hgrecco, could you reopen that issue so we can close it?)
• Executed black -t py36 . && isort -rc . && flake8 with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

[hgrecco]

bors r+

[bors]

Build succeeded:

• continuous-integration/travis-ci/push