Bump version and run towncrier for 0.6.0 release

Bump version to 0.5.2

Merge pull request python-trio#67 from touilleMan/release-v0.5.1

Bump version to 0.5.1

Bump version to 0.5.0

Merge pull request python-trio#45 from touilleMan/master

Bump version 0.4.1 -> 0.4.2

Merge pull request python-trio#40 from touilleMan/master

Bumpversion 0.4.0 -> 0.4.1

Merge pull request python-trio#38 from touilleMan/master

Bumpversion 0.3.0 -> 0.4.0

Bumpversion 0.2.0 -> 0.3.0

Merge pull request python-trio#24 from touilleMan/bump-0.2.0

Bumpversion 0.1.1 -> 0.2.0

Fix style