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

Start development of 0.17