Prepare 0.20.1 release.

Start development of 0.21

Move up deprecations in CHANGELOG.

Deprecations are generally important information and should be read
first.

Start development of 0.20

Merge branch 'master' of github.com:bpython/bpython

Start development of 0.19

bpython release version 0.18

Create release tag.

Start development of 0.18

bpython release version 0.17