v0.8.8

This is a minor release. In this release,

• Added Repo.copy_dir_all method, #53
• VersionReader can retrieve version information now, #56
• Repo can be destroyed using Repo.destroy method unconditionally, #57, #61
• Fixed weak reference issue for File object so it cannot be read after repo is closed, #59
• RepoInfo.uri method now doesn't mask secretive information in URI, #62
• Replaced TravisCI with GitHub Action
• Other minor changes and document improvement