Fix race condition in 'make distcheck'

When you run 'python setup.py test' it first writes to
check_manifest.egg-info/requires.txt, and then it reads from
check_manifest.egg-info/requires.txt.

If it so happens that Python 2.6 writes a requires.txt (that requires
argparse), and then Python 3.x reads that requires.txt, you end up with
an error:

  pkg_resources.DistributionNotFound: argparse

Moral: `python setup.py test` is bad and you should feel bad for using it.
Release 0.20
Post-release version bump