Bump to v0.5.0.dev3

- c42aeb5 Feature/more friendly development setup (#34)
- 2f9a71c New:Support testing in 3.8
- c8bd593 New:Stash unstaged code before testing
- d2a18a8 New:Best way to raise new exc
- 90fa9c8 New:ExprError __str__ implementation